So those are my heads and they really run well. Setup is currently a 445 @ 9.9:1, ported RPM, those heads, a 1000 HP Holley, Erson rockers, and a Bullet 236 @ .050 cam, 112 LSA on 106.
To answer your question though, modern valve job and cleaning up the guide area is the number 1 concern and will bring the most gains. However, the C8/D2 port is pretty small although it is on a med riser centerline. With a standard RPM intake, I believe it's significantly smaller than the intake. So if you look at a port's cross sectional area, you want it to slowly reduce as it gets closer to the valve, if you do not port, I assume (haven't measured) it won't do that and depending on your intake, you may have a bit of mismatch as you go big plenum, big port, step into small port, then a decent bowl, it'll run fine, but not ideal
That being said, I also ran a set of C8s with 2.15/1.67, completely unported, on my first 427 build in the Mustang. I never flowed them, but it wasn't very fast
If it were me, a modern valve job, clean up the bowls, and at least gasket match to a 1247 and blend as deep as you can.
