Rebuilding engine/ head gasket compression question

Flat tops, in general, should weigh less. No extra meat in a dome. The flat top will help promote flame travel. When used with a close chambered head, it should burn the mix very well with less timing. In the open chamber of the older "J" head, it will not matter to much.

The dished pistons are designed to lower the comp. ratio in any engine. This does not mean there not performance pistons. There very good to run in very samll chambered heads for lower ratios that will be OK for pump gas and super/turbo charged applications. These should weigh less than a flat top. But not allways.
You will have to check if it is a needed to know thing.

Once you have selected a cam to use, you should most certainly get the springs that are matched to the cam. This is extremely important.
I believe the cam you listed is the OE 340 cam. If you run over 9.5-1 with this cam, you might run into a octane problems. If you don't have the cam yet and are running a higher comp. ratio engine, look to the replacement 340 cam. It has a tighter centerline/overlap that will run better without pining.

If you have milled the deck of the block and/or the cylinder heads, you'll need to shorten the pushrods the like amount from there stock length.
Also, if you use a thicker head gasket than stock, you'll need to lengthen the rods by the like amount.

You have a total of .100 lifter plunger movment and the last .010 is not to be traveled in. This leaves you with .080 total room.

In example, you mill the block surface .010 and the heads .010, your down a total of .020. If you then get a gasket @ .040 (Fel-pro has a .039) your back to where you should be. Becaused you raised the head away from the block the same amount you milled off that made it closer.

Understand the plus and minus's gong on here?

Spendy ? Yes. But when it is all done correctly and the assembly is balanced well, you'll be very happy you did it right so you only do it once.
No engine build is a cheap thing to do. No matter what brand. Machine work is machine work and the machinest has to be paid.