Even early blocks are a crap shoot. I used a 72 model 360 for my 408 street/strip build in my 69 Dart, it ran 6.57 @ 104 and change in the 1/8 on pump 93. After five years I pulled it to freshen it up, the cam bearings and main bearings showed signs of block flex. I lucked out and didn't bust a cylinder and ruin a bunch of parts. Luckily I found and bought a complete but apart Indy headed R1 block engine. IIRC the R1 is around 60# heavier than a production block.