Guys, just a little background...
I've been talking to Jon (OP of the thread) over the last week or so giving him dozens of reasons to NOT do this project, but he is determined. He can score a stock set of 6.4 turbos cheap enough and has most of the hard parts to do this already. I think the biggest thing is going to come down to the tuning and the VGT control.
It's certainly not something I would try at this point, but maybe he will break through on this, he's pretty determined.
That said, I wish you luck Jon.
I agree, a 58V FICM isn't going to clean up larger sticks any better than a 48V. In fact I'd think it'd be worse if anything.
So the big turbo is a 65mm, with a tight housing. I'd stay under 205's personally, or go 205 with a small nozzle like a 50% (just read about someone doing that recently), that way you can just upgrade nozzle size if you want to go much bigger. Myself, I'd probably go 190/100%. That should be more than enough fuel for those chargers, based off of what the 6.4 guys are doing.