Get 2 wheels and tires in there that are what you want,make sure there equal and have clearence for the wheelwell ( plumb drops will make that easy ). Then measure from inside wheel mounting surface to inside wheel mounting surface, and then find a rear end that will work. If your going to put money into narrowing a rear end, go for a 9" Ford and get Chevy drilled axles..
Without a picture of the frame, it's hard to suggest suspension. 4 link/ 4 bar would probably be the best, but as a budget option, a ladder bar setup may also work, or the Nascar bar trailing arm type, that the '67-'72 pickups used.. hard to say what is best without a pic tho