Erractic idle and backfire sound like a bad set of points to me. If owner went to the trouble to rebuild distributor and not install a condensor, I have to wonder about the quality of the points and the rebuild of the dist.
A set of GM points and Condensor are cheap enough. Don't cheap out and buy a bargin brand. I used to work in several discount Auto repair places (national chains) and the points we sold were labeled "DELCO" but were low quality crap, If you really want to have a dependable Truck go to NAPA and buy a set of Echlin or other qualiy set of points and condenser. It sounds like you know how to adjust them. installing them is a snap. Dont forget to lubricate the point rubbing block, or dont bother. special lube should come with points.
Set your tiiming with a lignt. It is the only way to guarentee the timing is set corret. By the way, incase you dont know, changing point adjustment (dwell) changes timing. but not the other way around. Be sure to set point dwell first.
If still idling ruough, try choking carb with your hand or rag, If idle speeds up and smooths out, you are lean or have vacuum leak. Back fire thru carb could be vacuum leak, timing, condensor/points problem, advanced timing.
Let us know how it goes.
