I'm with the "stick with stock and modify as you go" crowd. There are at least 15 companies selling suspension modification kits to the stock frame. You can go as simple as beefing up the leaf springs, to lowered mustang 2 front kits, and 4 bar rear kits, or just add sway bars, etc. Before you get rid of the old frame check out classic performance parts, and no limit engineering just for starters! It's much easier to do this when someone has an engineered tried and true product with exact fits, then putting a chevy under there!
I know a lot of guys use the s10 frame for older coupes and hot rods and they work well for this, but check out the stores mentioned above before you make up your mind. Steve.