Double check my engine builder please 340 stroke

Wrong piston for what you want to do. Price is not an issue as the -20 dish are the same price from multiple vendors.

Doing the dance about elevation helping is folly at best. At 1000-1500 above sea level is not even worth the effort to try and sneak in under the wire.

Putting them down in the hole and running a.039 gasket you are just close enough for quench, but, that is getting iffy if the piston is more than .010 down. IMO, you'd want a .030 gasket to make quench work for you.

Cold days and high atmos pressure are going to be your enemy...