You said you had a 340 with flat tops and 13:1 compression.
Either you milled the crap out of the heads and block to do it or it wasn’t a 340.
My 340 is 12:1 (measured) and the pistons are .048 out of the deck and IIRC the dome is .140 tall. I mean the deck of the piston is .048 out of the hole.
With a 3.313 stroke its very difficult to get that much compression with a flat top.
So I was questioning either your actual compression ratio or your displacement.