OK !
Are all the cabs interchangeable over the years ('99 to 2013) or do certain years only work with same years ?
I know from 1999 to 2007 they are all the same. I have not physically put my hands on a 08-10 or a 11-13 to check but I have been told the 08-10 is the same as well.
Has the back panel changed over the years for cab vents or seat belt mounts ?
The 99-07 are all the same.Have the rear seat mounts stayed the same ?
Rear seat mounts also are the same from 99-07. One thing to note though if you plan to run the 40-20-40 seat like the front seat in all the back rows you will need to drill the holes for a few of the bolts. At least I had to in my XLT.
How are you going to handle the roof seam,
I was planning to step the end of the roof seam and weld one overtop of the other. If you look at the way it is done factory it will give you a better idea of what I am saying. I was planning to use the stock roof of the new rear section that i would be adding and joining it to the original cab roof. the headliner and the carpet ?
Carpet plan was to modify 2 factory carpet molds so that the seam would be located underneath the second row of seats. I figure with some time and patience I could get it to look like the seam was not even there.
Are you going to pull your cab to work on it (and the frame) or leave it on the frame ? Some shops seem to leave them attached, but I would rather weld on an open frame.
I plan to pull the cab. Actually I already have pulled the cab. I like the ability to get to everything without issue or interference. I plan to build a stand that will hold my current 4 door but be extendable so that when I mate the new rear section to it, it will hold it and help me with aligning everything properly.
How are you going to handle controlling 6 power windows from the driver's seat in a truck that was originally wired for 4 ?
My plan is to use the factory 4 window switches to control the original 4 front doors. I have plans to build a custom center console to go between the 2 front captains chairs and was planning to add 2 switches in that console to controll the rear 2 windows. I was planning to modify a switch for a standard cab truck to fit and give it a factory look.
Are you going to use a piece of Ford frame for the extension or a piece of common steel, properly sized/bent ? How are you intending to cut it or fish plate it ?
This is one place where my project might differ from yours. I am working on building a 6 door excursion, not a 6 door crew cab.(although most of the project is very similar except the frame) My plan is to cut the original CC frame below the rear doors before the frame rise. Then with the excursion I will cut the frame under the front doors before the rear doors and will mate it together using the front frame of the CC and the rear frame of the Excursion. I have to do it this way do to differences in the rear section of frame between the truck and excursion frame. The way I see it you have a couple of different options on how you could do it, dependent on if you have an entire donor truck to use or just a cab section.
1. Using an entire donor truck (ccsb ideally) you could make the frame cuts the same way I have layed out above and just mate the front of the CCLB to the rear of the CCSB. This would give you only one "seam" where the 2 sections of frame are mated together.
2. You could cut your frame and space it out the required distance and just add a piece of factory frame. I do not like this option mainly because it leaves you with 2 "seams" to reinforce.
3. You could cut your frame and space it out the required distance and just add a piece of common steel. I do not like this option mainly because it leaves you with 2 "seams" to reinforce and I don't believe it will look as factory as the first or second option.
As for how I plan to cut it, I plan to use a step cut i believe it is called. You cut half way down through the frame member, then cut horizontally for a few inches then down the rest of the way through the frame. I plan to reinforce it with a plate on the inside of the frame similar to how ford does the front sub-frame section on the trucks from the factory.
Are you going to do anything to facillitate better air flow into the back 2 rows of seats ? I don't think the dash air is going to cut it.
With the excursion air system I was not to worried about this so I did not give it much thought. I do know that the air system is the same for a RC truck as it is for a CC so idk of any upgrades you would be able to make to the factory system. You could always look into grafting the rear cooling system from an excursion into the rear section. Maybe underneath the rear most cab section or behind the seat? I will think about this some more and check out our company excursion tomorrow when it comes in my shop for service.
What are you going to use for seats ?
I plan to use my factory captains chairs for the first row. That will be followed up with a set of the 40-20-40 seats from the front row of another XLT that I aquired. I plan to use the factory 2nd and 3rd row of seats from the Excursion for my 3rd and 4th row due to the needs for a flip forward seat to gain access to the rear most row of seats.
I'm thinking of pulling my cab, stripping it except the dash and sending it to a body shop to have them install the extension and paint it. I'm thinking I'll handle everything else. How are you planning to do yours ?
I plan to do all the body work myself as from what I have looked at it seems very straight forward. The body sections are "modular" in design from everything I have seen so far, so they should go back together fairly easily. (that is my hope at least) I should say that I sources a wrecked truck that was already stripped down to the sheet metal to do a trial run on. I plan to pull the back wall off this cab and get an idea where all the spot welds are located and if there is any hidden things I may have missed so far.
What length of box are you going to use and how are you going to get it to that length ? Is it feasible to build an odd length box, like 7 feet ?
I am using the rear section of the excursion in place of my box so my plan will not apply here. Although I do have a few ideas. I believe it all comes down to what compromises you are willing to make. Is your number one goal to keep the overall wheelbase/vehicle length under a certian size? Or are you willing to add wheelbase length to have a certain size bed? For me bed space is more important that the overall wheelbase. No matter what I do I always find myself in a situation at one point or another where I wish the bed of my truck was bigger. For me an 8' bed is the only option. Also I dislike the sliding 5th wheel designs. I want mine rigid mounted to the frame so I know it is not going anywhere.
Have you priced anything out yet ?
I actually have been collecting parts for this project over the last few years. A little here and a little there as money permits and I could find deals that made the purchase worthwhile. Currently I have all the pieces that I believe I need with the exception of the Excursion. I have not found one in the price range that I am willing to spend in a condition that is satisfactory for me yet. Once I do find it I plan to start this project as I am getting antsy and tired of waiting 
