I have noticed that a lot of you have replacement rear ends that are 3 inches shorter than stock on each side (shorter than stock by 6" total). Is that the max that a rear end can be shortened while still using the stock suspension?

 

I suspect 3" is probably the limit, but you could determine that by measuring your stock rear. Look at were the brackets at the end of the axle would be relative to the suspension with 3" of the axle tube removed.

If you are asking about a 10-bolt, some people will use the rear axle assembly from a 3rd Gen F-Body, which (according to the dimensioned diagram from the Strange Engineering website, and other references) is ~2" narrower per side than the 4th Gen rear axle assembly. Strange has direct bolt-in units for 12-bolt, Strange 60 and Ford 9" in both 3rd Gen and 4th Gen dimensions. The 4th Gen rear allows a wheel backspace of a bit under 8", the 3rd Gen will allow up to almost 6". Narrowing an additional 1" per side would make the maximum wheel backspace almost 5".