how strong is the 8 3/4 #741 case?

The main problem with a 741 is lack of choice of new gear ratios for one. Only 3.55, 3.73, and 3.91, as far as i know.
A 741 should live a long happy life behind a 360/torqueflite combo. An idiot with a stick can usually kill anything, given enough opportunities. Even with a 350hp 360 and a fourspeed, Id worry more about u-joints than the 741.