[unruhjonny]

wow - Great find!

I'd find some cheap pre-1970 small valve heads with 72cc or smaller chambers - those can often be had for a song.
I think some were being offered for free in a recent Smoke Signals - but location would determine if that might work.
If you go for some thing newer than 1970 just be sure you don't get a "400" or worse, a "455" head;
Stay away from 4X, only do 6X-4 (not 6X-8), 1973 #46 heads can be had for super cheap, and 71/72 350 heads are also generally give away heads.

I think four valve reliefs are standard fare for all but custom pistons these days;
From a quick google search those appear to be a forged slug.

I have long been a proponent of the P350 - I get why people have traditionally tossed them in favour of rebuilding a P400, but they are great motors for what they were, especially when stacked against a bread and butter same year Chev 350.

Good luck!