2”. Forcefed86 on the turbo forums made like 1000hp with 2” fully divided hot side on a 4.8 and went 160mph to the 1/4 and said spool was much faster than 2.5 or 3”. If your not fully divided though I’m not sure how small you can go.
1-5/8 is a good size for 318 tube header primaries, so sounds interesting, however
4.8 LS doesn't really prove it. I'd like to see you build one with larger tube's & test them back to back, only changing pipes. It sounds real good for street use, which is what I need, but for 1/4 mile, will it work @ higher rpm or lay down?