This is what we THINK we've found out..... lol

79" total rear body width. I can get away with 76" outside of tire to outside of tire.
With a 295/65/15 on a 15x10 rim with 3 1/2" backspace we figure we need a rear end 59 1/4" overall width.
Springs are 43 1/2" center to center apart.

A 1966/1967 B body rear end is 59 1/2" overall width and 44" spring centers......

Hmmmmmm

I'll just have to split the inner can on the bed and move the inner to the next raised bed section, about 3 1/2" inches inboard, toss a strip in and make it pretty and nobody should be able to tell we monkeyed around....
An 8 3/4 is .65" longer center of axles to center of u joint, and my driveshaft looks a wee bit short anyways, the swap will probably HELP?! lol