Yeah I was going to say that if they do bolt up (Which im pretty sure they should, I dont see why they wouldnt, though I wasn't 100% sure so I didn't post) they will have a lower C/R than the old heads. Look at the casting numbers in the heads and you should be able to google the combustion chamber size and compare.
I still think it would be best to just drop the 5.0 in there. You could still run a carb, just swap out the intakes. The 5.0 will have a roller cam and revised firing order. You would also have to install a fuel pump eccentric. I would imagine the 289 cam cover would swap over to the 5.0.
Mustangs Plus - Tech Articles - Project EFI GT (Page 1)
That is a pretty good article on swapping over to EFI.