need the opinions of a few 340 guys

You will be fine.

Ok

With your piston sticking up .021", you have a negative 4.5 cc

You will need a total of 74.3 cc for clearance volume.

Clearance volume is the; Volume of the piston + volume of the head gasket + volume of the combustion chamber.

For a .040" fel pro head gasket the volume is 8.5 cc.
volume above the piston is - 4.5 cc (because it sticks up)

Now you subtract.

74.3 cc + 4.5 cc - 8.5 cc = 70.3 cc


So if you use a Fel-Pro .040" head gasket with a volume of 8.5 cc, you will need your heads to have a 70.3 cc combustion chamber to reach 10.5 compression.

Most 340/360 heads start at 72 cc. You need 70.3 cc for your compression. So if your heads are "stock/never decked" at 72 cc, you will want the head guy to mill them down to 70.3 cc, or take 1.7 cc out with milling your heads to get 10.5 compression.

Have your heads cc'ed to see where they are at. Your target is 70.3 for 10.5 compression and a .040" Fel-Pro head gasket.