From my understanding if you have to big a tailpipe you actually lose power because there is no back pressure. I believe that for a 350 you would want like 1 5/8 headers into a 2.5 inch exhaust to mufflers. past mufflers you can run 2 inch since the exhaust gases are cooler at this point. I think it was due to hot gases expand. I'm no exhaust expert but this is what several other hotrodders have explained to me. also you would get extra h.p. if you add a crossover pipe