what hp can a 742 case 8 3/4 handle

Add one of these
http://www.markwilliams.com/detail.aspx?ID=609
Brace the housing and beat it with impunity at your weight and h.p.
A 9" or a Dana would be a better option for various reasons , but the 8 3/4's are plenty sturdy if you do these two things.
The difference between 489 and 742 is not as huge as some might think.
With the crush sleeve eliminator , it is apples to a little smaller apples.