Measure Measure Measure, there is a good calculator on Dr. Diffs page. Something to watch out for, my wilwood disc brakes added almost 1 inch to the track width because the aluminum hats were .500 thick. I didn't even consider that originally and luckily caught that before I submited measurments.

I went from a drum brake 8.75 to a disc brake Dana. I knew I needed to narrow it just a small amount to help with tire clearance so I did. But I did it wrong, my rear end setup guy fixed it for a very fair price, but he fixed it wrong and had to do it again. It is all good now but I think at one point each of us was frustrated with the other guy haha.