[Aussie Mike]

I went with 1 7/8" primaries that step up to 2" into the side pipes and the collector is 3 1/2".

I went the larger 1 7/8" because I thought I might supercharge the motor later. I reckon Phil has the ideal for a Naturally aspirated LS1 with the 1 3/4" as they will give pretty good all round performance with room for upgrades like a cam and heads.

Cheers