To the op glad to see you are sonic testing it, I saw a virgin bore 340-1 block sonic checked and one of the major thrust walls was only .130 thick. The only blocks I have seen that consistently test good/thick everywhere are the 70-73 360 blocks with casting number 3418496.
I have a slant 6 block I just sonic tested and it is almost 3/8" on the major and minor thrust sides and over 1/4" on the opposite sides. All 6. Stupid thick.