Only AndyF would know FOR SURE, but I don't think the drum spindle changes anything because the bearing dimensions are identical to allow using the 73+ rotors. That said, I don't think I compared the two side by side when I did my swap, but I did run the 17's before the Viper swap and didn't note any clearance issues. Going to a short zerk fitting was all I needed to clear the UCA/UBJ, and the OTRE relationship didn't change. Makes sense, due to the same LBJ being used on each system. Just speculating, but I think the 73+ drum cars were still SBP, so the Cobra wheel woudln't swap.
Not sure, but I'm guessing it would be better - sounds like a good project. How much, I don't know. I typically believe you should stuff as much tire under your car as will fit and function, but that may not mean going to larger wheels. Some of the stock-class GM F-body autoXers do pretty well pinching a 275/40 on to an 8" wheel...