I used 3.5 heads on my 4.5. They were a straight swap. Used the m117's manifolds of course - a 3.5 has the same exhausts I believe, but a narrower intake and a smaller diameter throttle body and no idle air circuit. I measured ONE cylinder (#6) for compression after this and got 145PSI, or about 9.9:1 compression ratio. I am assuming it's 10:1 and I have some compression loss in my cylinders (it might even be more, my block has 145K on the original bottom end). It drastically increases the top-end power as well as off-the line power (which I missed at first due to a misadjusted mechanical advance I fixed last week).

Only problem might be: The 3.5 heads were for D-Jet EFI, the 4.5 block you have was a K-Jet and your setup for it (if you have it) is K-Jet. You might need to search around for the correct O-rings or whatever to secure the K-Jet injectors, I don't know the difference between them and D-Jet ones at the base.
EVERYTHING about the 3.5 heads is identical to the 4.5's except the combustion chamber. See my adventures here:

http://www.peachparts.com/shopforum/showthread.php?t=123638
http://www.peachparts.com/shopforum/showthread.php?t=124227