Just as getting the engine bolted in the car is about a quarter of the work in an engine swap, I suspect that just joining up the appropriate wires is simply going to be the start of making the electrics/electronics work. Like Chris, I would want all the systems to work, if at all possible.
It doesn't take much for things not to work; I found that a Sierra temp gauge sender does nothing when fitted to a Capri, even though they were visually identical. I know that traditional rev-counters read incorrectly when fed from a different number of cylinders; I suspect that's going to be the case here.
I'd want to know that the ABS/TC ECU won't have the raging hump when divorced from the engine, before going much further. If it just powers the speedo, you'll be ahead. Re-programming is unlikely to be something you can do at home!
Unlike mechanical issues, the electronic ones are likely to leave you shouting in rage at something that should work, but doesn't. Well, they do to me.
You mentioned that GM's prototype had compromised headers, due to the lack of space, I'd be amazed if what you end with isn't compromised too. It amazes me how just fitting one more small component entirely fills what looked like a big space. The trick to this is to fit as much as you possibly can before committing to any difficult jobs. The steering and headers are definitely going to be difficult as they both have to be in particular places.
At least you're going into this complicated job with your eyes open, unlike some of the daft stuff I've done in the past.