The track width on that el camino is 58" with a wheel base of 116" (sharing the same wheel base as the Malibu Wagon. That is one of the G-body GM metric chassis (two wheel bases, the shorter being 108"). The frame perimeter is wide, about 54", riding along the rocker panels in the el-camino, so it would sit outside of your cab. Remember, the El-camino was a car that wanted to be a truck, not the other way around.

Here is a pic of the stock GM metric frame (108" wheel base shown) http://www.thedirtforum.com/Metricframe.jpg

You could use parts of it, however would spend more time than making a new frame from scratch.

The engine is most likely a 305 as they were one of the 4 available engines (Buick 231, Chevrolet 229, 150hp 305 and 165hp 305)

Since the 700R4 and 200R4 were not introduced until 81, you most likely have a TH350 in it

Hope this helps

The problems we face today can not be addressed at the same level of intelligence we were at when we created them - Albert Einstein Or with the same level of $ - Me