1970 340 has the 1973 low compression pistons

Rat Bastid said
You’ll be lucky if what you have is 8.0:1 and that’s low.

The pistons don't look like they are very far down from the deck surface, I'd say closed chamber heads with around a 62-64cc chamber and 0.039 thick head gaskets should put you around 9.0:1 - 9.25:1.