There are a couple of conventions to be aware of regarding exhaust specs. Length is specified on the center line, and diameter by the OD of the pipe, with 18 ga. being the standard thickness. A 1-1/2" pipe (just fine for a hot street engine even in a 750 cc. displacement) will thus have an ID of just over 1-3/8" (or just at 1-3/8" if 16 ga. material is used). Tuned length, however, is specified from exhaust valve to outlet, again measuring on the center line. Michael Morse shared a rule of thumb with me awhile back that seems to work pretty well; on these engines, measuring the pipe on the outside line gets you very close to the tuned length.
Guys, let's not assume that anyone wants to pick up the responsibility of administrating a group buy on a bunch of pipes. With collecting funds, shipping, and so on, it can be a major PITA.