Any guidance on my 1970 Duster plans

[DionR]

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.

If you want to run a cheaper 17/18" wheel, the wider B-Body axle makes it easier as most late model wheels have more offset built in and a narrower axle would require spacers or a custom offset (negating "cheap").

The other advantage when running 17/18 wheels is the larger diameter allows for a wider front wheel and tire to fit inside the fender when more offset is used. And the B-Body axle sometimes allows for running the same rear wheel and tire if you want to run a square setup.

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?

There are 2 reasons to do this, in my opinion. Sometimes the 1/2" offset makes or breaks a cheap option for a rear wheel. The other is if you want to get maximum rubber under the car. All A-Bodies have the inner wheel well about 1/2" inside of the spring and moving the spring in means you can use more of the wheel well without minitubs.

I wouldn't worry about moving the springs, Dusters have large wheel openings and you shouldn't have a problem with rubbing.

While the Duster/Demon is better than the other A-Bodies, they aren't as generous as they could be. Thus the reverse tub the OP is talking about and @72bluNblu did. The factory left the wheel well pretty much the same as the other cars but with a notch. So if the Duster/Demon is lowered, they run out of room for wide tires before they should. Thus, a 1/2" spring location still helps even if they don't need it as bad as a Dart or something.

Here is one of Blu's pictures from when he did his reverse minitub. You can see how the tub doesn't come all the way out to the fender like they could have done.

 

[DionR]

I would add that when the factory widened the A-Body in '67, they left the rear track alone so when stock they look funny and really narrow (to me). The factory should have put a wider axle in the car when they did it, and a '65-67 B-Body axle is about the right width even if you don't plan to run 17/18" wheels.

Just my opinion.