Did you try the backing plates and drums off your 7 1/4"
I used the entire brake assembly of 1974 7 1/4" on my 8.75
I did use Dr Diff axles in mine $300, bolt on done deal.
No I didn't think to try that. They were 9", but if they would have worked I would have been ok with that. It's something to think about though. When I get back if the ones he shipped don't work I'll give it a shot. I think it would at least need an extra hole drilled but the rest I'm not sure about.