I have a 91 z28 that has the axle casing cracked. its a 342 gear. I have access to 86 iroc rear end with disc brakes. the question is can i just bolt on the 86 rear end to the 91 rear end without doing any mods? Thanks in advance.

Not sure that I understand the question. Are you wanting to remove the '91 rear end and install the complete '86 rear end? If so then yes you can. If you have drum brakes then you will have to do a few modifications to make the disc brakes work right. If you are wanting to take parts from the '91 rear end and put them in the '86 housing then you are getting into a big job and a lot of set up time.

yes that is exactly what i want to do is install the complete rear end into the 91. Also the 91 had a 342 gear and the 86 has a 323. How much should that effect the performance. I have a T5 5 speed lb9 motor and was running 13.60 in 1/4 mile. Both have disc brakes also. Thank you so much for the quick response.

The '91 has 28 spline axles and the '86 has 26, so you are going down in strength slightly. The 3.23 gear isn't going to be a huge difference in performance, but your speedometer is going to be off some.