what hp can a 742 case 8 3/4 handle

The 742 is a strong unit. I've seen it handle as much as 700HP. You should be OK if you just launch with the foot brake. I've used the 742 season after race season. At the end of the race season, I always rebuilt it with new gears and clutch pack. Of course if you increase your horsepower much more you'll need the Dana 60. JMO