stock 6.4 turbo sizing is not your problem. 650+ hp can be had on a 6.4 with stock injectors and the Elite tow power atmosphere turbo setup, and close to 600 on totally stock turbos and injectors. I just did some checking on the 6.4 pedistal for you on a 6.0 block. to answer some of your speculated questions, oil drainage will be easy, but will require machining or drilling the block where the pedistal goes. you just have to drill straight down through the flat part where the 6.4 pedistal will go to make it drain. It will just go down inside the block from there.
The big problem is how to drill without any shavings getting down in the oil, so to do it right, it must be done with the engine ripped down to a bare block. You'll need to do this for the pedistal mounting bolts as well since none of them come close to lining up. Drill, and tap the block to get the pedistal to mount and allow drainage. I suppose you could cram enough rags in there to make them catch all the shavings if you remove the hpop to access the space, saving you from ripping it down to a bare block to keep out the shavings. It's not something I'd be willing to risk on a project though. You'll also have figure out several more things to get the pedistal to fit properly because the block is not machined smooth where the pedistal should mount, which needs to be a sealed surface. There is also a fitment issue as the 6.4 pedistal is too large to sit down between the oil cooler and HPOP cover. Clearancing of the pedistal (which will destroy the 6.4 pedistal gasket, or eliminating mounting bolts and feet of the oil cooler cover and HPOP cover are required to allow room for the pedistal. You'll have to come up with a way to seal the base of the pedistal to the block all on your own. I didn't look, but the 6.0 block bolt holes for the stock turbo pedistal also might cross paths with the 6.4 pedistal gasket, further complicating your sealing requirements. Welding the factory turbo drain port shut on the HPOP cover should be no problem, but it's something to think about as well.
The good news is that once you mount the pedistal, 6.4 stock up pipes should bolt right on using 6.4 exhaust manifolds as the pedistal fitment front to back will be in the identical location as it sits on the 6.4 block. The same applies with the 6.4 down pipe. It should bolt right on. You'll obviously need to change the flange at the bottom to accept your existing exhaust, but hey, it could work. As far as your intake goes, I think you'd be best off to attempt using the front of a 6.0 intake, and itegrate the back half of a 6.4 manifold for turbo clearance and keeping your alternator mounting. The only other feasable location I've seen to mount an alternator is the other side of the top, or in place of the a/c compressor, which is a real kill-joy for daily driver trucks.
Of course you still will need to make room by relocating your FICM, and possibly the coolant reservoir to allow for the factory 6.4 compound piping with a custom mounting bracket as none of it will interchange.
Can all of this be done? Sure, but is it worth it? We at Elite decided long ago that for marketing purposes it is not simply because of the block machining requirements to make it all happen aside from everything else. We're working on a conversion kit so guys can drop in a turn-key 6.4 kind of like de-stroked does with cummins stuff, except for the part that a 6.4 will already bolt in, and it's just a matter of electronics, piping, plumbing, and stuff like that. That's our approach to getting the 6.4 turbos under the hood of a "6.0", plus the benefit of common rail injection. Good luck. I'm curious to see how the pedistal problems are resolved. BTW, do you even have head studs yet? if not, you definitely will before the turbos go on.
