340 question

Yes and no. While a cast crank is a down grade, there still plenty strong. The low compression pistons are as you say, easily changed out. Even so, a set of closed chambered heads will raise the ratio to a reasonable level. You'll need to know exactly where the piston sits, the head gasket dimensions, valve relief amount and head chamber cc amount for the calculators. Getting to a 9.0-1 should be easy enough. More is better. But lets see where you end up first. At 9.0-1, thats enough for most street cams to perform vcery well.

Who was the guy with an OE 360 short block & W2 heads? Running low 12's IIRC? It is a great and cheap combo. He invested in heads, cam and valve train. BAMM baby!
As one of my engine shop owners sez.."when is the last time you saw a Mopar crank break....never".
.and he's a Chevy guy.