You're kind of in a pickle.
Using the engine size, a zero deck piston, 5cc in the valve pocket and 64cc head, you're right at 12:1.

First of all don't buy the Edelbrock 340 heads. They were made for guys who already had new or nearly new 340 pistons that come out of the bore. By carving out the material where the piston would collide, they eliminate the opportunity to get squish.

For a 30 over 4" stroke 340, to get to 10.5:1 you need 90cc above the piston. Assuming your new head is 64cc, the head gasket is 10cc, there's 5cc in the valve pocket, you need about 11cc more volume. This comes out of the piston top closest to the spark plug. That's with piston at zero deck.

To get to 10.5:1 without machining the piston, it needs to be down in the hole 0.046".

Regarding heads, the consensus opinion for the best flow out of either an X or J head, stock, is 220cfm. The max flow out of a Edelbrock LA head is 251 cfm. Using this as an example you are looking at as much as a 65hp increase.

A 3" diameter circle 0.090" deep is about 10.5cc.

Clear as mud, right?
Rats! I just found out that while I was calculating a 64cc chamber, the LA head is 63cc.

R.

The Magnum heads are only 58cc and would need to have an inverted dome machined into them that matches the chamber's shape. Just slapping them on with a 10cc gasket, piston at zero deck, and 5cc valve pockets you'd see 12.7:1 compression.