I got a heads up on a 1988 gm Chevy truck with aluminum box mounted on it and want to know if you have installed these before? Is the bottom of the box two i-beams? how are they bolted to the frame?

Take a look at this 88 Chevy. I do not know its sub model. What would be the typical spread of its frame ie what is the distance center line to center line between both frames? I may buy this truck, and remove the box to install on my truck but do not know its frame spread.