Sonic Testing ( LA Blocks )

If you are going to sonic check blocks, do the 68 & 70
First.
I have read mixed reviews on 68, (either epically capable, to fairly thin on a .030" over bore) & have not read of failures regarding significant over bore of 70 SBM's.

Ryan Johnson of Shady Dell has a write up somewhere on the SBM's,, Muscle Motors and 440 Source on the BBM's.

My non-expert opinion/guess after reading is that the 70 is your best bet, with the 68 potentially being thicker.
Not sure how much sonic checking will run you, hopefully this saves you money.