3". I believe any neckdown aft of the header collector restricts flow & therefore hurts power & you want no restriction & the least amount of backpressure from there rearward in a 4 stroke eng. Gas speed/scavenging is a seperate pri tube dia/length issue. But a 3" setup is going to be VERY loud & you'll need every ex trick in the book to make the noise liveable, X pipes, addit'l glasspack muffs alongside the leaf springs & maybe add some sideways capped 1.5 ft long pipes welded to the 2 bends over the axle (helmholtz resonators iirc). Ain't tried it but reportedly it is an accoustical godsend for noise reduction. Best might be those trick muffs that open up when you get on it I forget what they are called or some cable or electric operated cutouts along with restricted muffs (keep em quiet till the money is in the hands of a neutral 3rd party ). 2.5" would be troublefree. 3" would be a certain amount of gain but would be alot more work/planning. Mandrel bends everywhere.