The stovebolt bellhousing is attached to the block with 7/16-14 thread bolts. There's no way to get a 1/2" bolt to fit unless you drill and tap the block for bigger bolts. I just finished bolting the adapter for an engine stand to a 216 block, and 235's and 261's use the same bellhousing bolts as a 216.

Truck transmissions (SM-318 and SM-420) are bolted to the bell housing with 1/2-13 bolts.

Standard thread bolts (UNC) are as follows:
3/8-16
7/16-14
1/2-13

Fine thread bolts (UNF or SAE):
3/8-24
7/16-20
1/2-20

Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!