i like 2inch for some thing 36 long primaries that 450 and 3 .1/2 collector if you run 3. 1/2 ex. 1.7/8 is ok . this is not big at all for a big block chevy. to make them run you need to unplug it .so i wound try to keep away from the small tub headers. on the engine you drop it down? why?