They will crack when the engine "kicks back" due to over advanced timing ,and not using the front brace to the block also encourages cracking,though few people ever put them back on (or had one there to begin with)...
When I swapped a SM465 in place of a TH350 in my '72 K5 I had to use the cast iron nosed starter because that was the last year they used a cast iron bell housing ,that the starter bolted too,instead of bolting to the block...after '73 they started using aluminum bell housings and both automatics and manuals used the diagonal bolt pattern starters that bolted to the block...
I'm pretty sure all truck engines had the 168 tooth flywheel or flex plates,some straight sixes and 307's sometimes had the 153 tooth flywheel and a straight across bolt pattern starter..
I have a '73 407 in my van that has the straight across bolt pattern starter..(and I assume it has 153 teeth on the flex plate )..
When I swapped a SM465 in place of a TH350 in my '72 K5 I had to use the cast iron nosed starter because that was the last year they used a cast iron bell housing ,that the starter bolted too,instead of bolting to the block...after '73 they started using aluminum bell housings and both automatics and manuals used the diagonal bolt pattern starters that bolted to the block...
I'm pretty sure all truck engines had the 168 tooth flywheel or flex plates,some straight sixes and 307's sometimes had the 153 tooth flywheel and a straight across bolt pattern starter..
I have a '73 407 in my van that has the straight across bolt pattern starter..(and I assume it has 153 teeth on the flex plate )..
