If you want to do a head swap to lower the CR, you want 6X-4, 5C from a 400, 96, or 7K3. The 7K3 will need manifolds or adapters because the end exhaust bolt holes are not there and there is no material to drill. They are big valve heads with guide plates. The 96 is from a 1971 400 and they are big valve low compression heads. The others have a 1.66" exhaust valve, which really doesnt hurt much on the street.
If you dont mind getting some work done to the heads, you can use certain 350 heads. The 1971 vintage 350 head, I think the casting number is 46 or 47, have the right size chambers for just over 9:1 on a 455, but they are small valve heads, some have screw in studs some dont. You can have them ported, bigger valves installed and screw in studs installed if they dont have them. They will work just like any other D port head that has been ported.
Also other heads that have pressed in studs but will work are 15 heads, they were the low performance 455 head in 1970. All of these are D port heads, and while round port heads will work great, even the HO heads they are more expensive and require more expensive headers or manifolds.
You want around a 90cc chamber, give or take a little depending on deck height, to run pump gas. I had some machine shop ported 5C heads on a 455 and it ran very well on 87 octane, no ping at all, and it had a 224-230 cam in it with a Qjet. 13.50s @105 with 2.93 gears, stock converter in a 4100lb car with traction issues isnt too bad for a low compression engine.
Those 64 heads will work great on a 400, puts the compression right around 9.5 with a zero deck.
Otherwise, pull the engine down and install dished pistons. To me a head swap is easier, and a fuel change is easier still, if you can find it.