Any guidance on my 1970 Duster plans

Ok, some progress, I did manage to get a 73 V8 k-frame, as well as LCAs to go with it. That stuff is out getting cleaned up and degreased so I'll be starting to weld reinforcements on those in the next few weeks. My wallet is a lot lighter as a bunch of stuff has been ordered, with a bunch more yet to be ordered. Been creeping around here reading and trying to learn where I can. One question that I have, I notice a lot of guys run b-body 8.75 rears for autocross builds. Why is that? Whats the advantage. I picked up what should be an a-body 8.75 at Carlisle last year not quite yet knowing where I was going with this car just that the original slant 6 rear was not going to work out long term.

Also, I'm starting to think about tires and brakes now too. I suck at wheel/tire planning, still a skill I'm building for sure. I notice there is a lot of effort put into the 1/2in leaf spring relocation. Does that really make that big of a difference? I looked into it, and it seems pretty straight forward with the exception of moving the spring perches on the rear, which may be beyond my ability right now. I have a better shot at pulling of a reverse tub like @72bluNblu did, but just barely. I'm going to have the thing painted so I'm trying to figure all this out before it goes out.

I've got the suspension figured out for the first iteration (well, good enough anyway, just need to keep buying stuff for now) and I really appreciate all the help and suggestions. Once anything interesting happens I'll throw some photos up, for now its just outside in the snow.