On your first question... can you use the old pistons to see where the deck is... yes, but you will need to measure from the top of the pin bore to the top of the piston accurately (a vernier caliper will do) Then you do the four corners and you have the formula: Deck hieght = 1/2 stroke + 6.123(rod length) + piston measurement + 1/2 piston pin diameter + distance piston is down in the bore from the deck (use a straight edge and feeler gages).
My impression is it will be down in the hole at least .030 and maybe as far as .080 depending on the year it was rebuilt to replace. On top of that you have 340 heads that will typically start at 72ccs and might be as big as 74. All this means basically there is no way even with the domed pistons you will have a hard time breaking SCR or 9:1. Which means the DCR won't be anywhere near the 9s, or really the 8s depending on the cam you're thinking of.
