Will a 1986 Camaro Z28 Iroc rear end work on a 1972 Chevelle?

Nope, completely different mounting brackets... They use a torque-arm and panhard bar, chevelles use a 4-link type suspension.

they also have the 7.875" (?) rear which is really weak.

thanks

While an F body will not bolt in, a G body rear will as I have done it to roll a chassis around. The only thing is, the 7.5" rear is the weakest rear GM ever built. You would have to find an 8.5" rear from a 442 or a Grand National and they are major $$$$.