can a posi rear end from a 83 camaro fit in a 85 elco

the size is right, it depends on the splines though.

If I'm remembering right it will not fit. 83 Camaro's use a pan hard bar to keep the rearend centered. Elky's use a 4 link configuration. It would take quite a bit of fab work to make one fit.

oh, wait. you want to swap the whole rear end? its too wide. The carrier will fit though. Camaros used 26 spline from 82-89, then 28 from 90-92.