I have switched rear ends 3 times. I now have good information for all of us doing so.
1986 IROC 2.72 drum open width 62"
1987 TA 3.08(believe me that is what it is) posi disk width 62.25"
2002 SS 3.42 PBR Disk posi torsen width 65"

The difference is not 2 inches between rear swaps it is 1.5" or 1.375". This is good to know if you are trying to put 17X11s on your car like me. I have not tried to figure out the offset for the ZR1s. I know that there are 2 different ones, does anyone know which ones fit the later style rear end in a 3rd gen.

 

The difference IS 2" per side if you use a 93-97 4th gen rear. LS1 rears (98-02) are just under an inch shorter.