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

If this is really a "what if" build I think you'll be fine.
The factory wouldn't tub the car so probably limited to A12 size wheels in '69.
Which I'm pretty sure were only 6" wide.
So, I'd stick with the stock wheel size, which should limit traction, and look more 1969!
And I'm not an A833 expert, but isn't there a 18 spline, "big yoke" version you could build?
I'm assuming this is what the '68 Hemi A cars used.
I believe most A 4 speeds used the small yoke.
Adding any external bracing to the rear would spoil the '69 look.
The factory would have probably run the Dana, not sure how wide the Hemi a cars were. Sounds like a neat build!