A stock B13 case can fail with a Stock BlueBird DET powering it or a simple 100 shot of NOS on a DE with all of the suspension & mount upgrades done. The back of the case will crack and break off if the 1/2 moon crescent if not reinforced.
3rd gear will fail most commonly from torque which twists the case over time. I had a trans that after several 3rd breaks we finally replaced the case and then 3rd lasted almost 9 months before failing again vs the previous only lasting several weeks.
Manual trans coolers have been done and do help. Mount and electric pump below/besides the trans. Suck from the drain plug go through a cooler and back in from the fill plug is easiest. In most circumstances, this is only needed for road race applications as the brass bulk rings tend to melt from extensive and pro longed heat.
Wheel hop is a killer, slicks are a killer, big torque is a killer. Bottom line, the trans is a big weak point. Lots of people think that they will not be happy until they are making 450~500 hp for a street car. IMO, 300 is plenty in a B13/B14 for a fun daily driver.