I'd like to chime in and say thanks to those who weighed in on the Rover V8. Years ago I'd seen some pics of some rather expensively done up 944's in the UK with the swap, so it was what immediately came to mind. Seems to only be a viable alternative to a 'Vette engine if you're willing or want do have a "built" engine instead of simply trying to get it to fit.
If performance is your goal, it really is hard to beat the lightweight modern 'Vette powerplants.
I may be taking apart/rebuilding/upgrading the engine in my '84 944 mostly for the experience acquired along the way. Going into it - I do not see it as a sensible thing to do, but frankly, if we were all sensible, we'd live and work in the same place, and not drive at all.
As for the post made above mine, you'd want to be good with a cutter/welder.
Yes, it could be done, probably best to cut the rear out of the car and put in a subframe, locating the engine in the back half of the car. I doubt the width of a horizontally opposed engine would fit in the space of a midly slanted inline 4. You could of course, go super wide body with spreading out your strut towers with totally new fab work. Not impossible, but perhaps not practical.
The 944 kinda is what it is if you want to stick to Porsche powerplants. The 951 will be a lot faster for similar cost.