The monte ss steering box will fit, for sure, any monte carlo, malibu, el camino from 1978 to 1988. As far as pontiacs and olds go, I haven't a clue.
I put one of these steering boxes in my 1980 malibu and it was a direct fit as well. Your el camino probably used the same junk ring retained housing my malibu had from the factory. You don't have to do any mods, it is a direct bolt and play deal. The only problem I had with bolting mine into my malibu was that my factory pitman arm was .010 smaller than the monte carlo's pitman arm. So I just went and pulled one off your everyday regular monte carlo. Trust me, your steering will be stiffer and feel a lot better.