another question regarding 75 roadrunner application. lookin to replace 8 1/4 rear dif with 8 3/4. is the 71-74 "b" body the same width overall, and from leaf spring perch to perch? I have found info that I believe both are same width, but not certain on perches...figured id check to see what some of the members may know. thanks

Have a look in the Tech Archives. The link is at the top of the page. From what I found, the 71 B-body has a 63" flange-to-flange measurement. The perch measurement(center to center) is 47.3".

Perches might be different due to the iso-clamps?

You're probably right. When I put an 8 3/4 in an 82 Cordoba, I just drilled the pilot hole in the perch to the larger iso-clamp size. You can also do away with that system completely but then you'll need some of the old style shock mounts.

I had a 75 Fury and put an 8 3/4 out of a 1974 Roadrunner without any problems that I can recall

The 75 Road Runner is identical underneath, to a 73-74. Yes, a 71-72 8 3/4 will bolt right in, if you ditch the iso clamp parts and use the 72 and older shock plates and U bolts. Even better is to ditch the 75 rear springs, with their oval eyes, and put a complete rear suspension from a 71-72 B body in. That way you get stiffer front spring bushings too. The rear springs on 66-72 B bodies fit right in if you use the early spring hangers too. So you could put in HEMI springs if you want to.