How much hp/torq will an OEM 833 4sp and 8 3/4 take?

Clutches break parts. You can put a bunch more through a 4 speed and 8.75 than you are going to IF you get the right clutch.

IMHO, that would be a Sof-Lok from McLeod or similar. They are totally street able and won't break your parts.

They aren't cheap either. But neither is breaking parts.