Probably because the earlier 340s absolutely ripped. (I recall the NHRA factored one 60HP over the rating.) 1968 (only) 4-speeds got a big cam, and 1970 used the J heads, big carb, and high compression.
Unless you swapped the pistons, I would guess you were down most of 2 points of compression in that 360. They were all low-compression engines...I would bet that whoever rebuilt the 340 used higher-compression pistons.
It's not complicated...new design, made in China. Old design made here, will eventually be dropped. US factory closes when all tires made there are dropped. Gradual transition to making all tires in China.