Good deal flattop, you'll enjoy enhancing your program. The ID of the pipe is really more of a trial & error thing. We really don't know the gas/pulse speed in a given pipe unless you find the best one at a certain RPM and run the math to check the speed versus the length. Some time a smallish diameter pipe will run well for a few laps or seconds on the dyno,, then when it get's up to full temperature the performance falls off due to the hotter gasses being too fast and the pipe becoming "out of phase". Also a too short pipe may run fine with a too large ID because the slower gas speed is "in phase" with the shorter length. Or a too large ID that is rapped runs good after a few laps or 25+ second dyno pull. You may find a "slightly "smaller ID pipe will pick up TOP end power due to speeding up the gasses acting like a shorter pipe.
You'll find an engine with a short rod ratio, like the stock Briggs FH, likes a larger ID pipe than the same engine combo otherwise with a long rod ratio. The longer rod ratio affects piston dwell at TDC being more efficient. I notice the Clones run small ID pipes, my guess is they have a rod ratio in the >1.8 to 1 range and a more efficient combustion chamber. Just a guess,, haven't worked on one.
In the winter of 1993 when the WKA went to the RLV muffler on a national basis for the 1994 season, we did a lot of pipe testing. My son was running the FH .425 Purple plate JR1 class. These engines had a narrow sweet spot and just having more power was huge. After many iterations our best "home made" pipe design was tuned to the power peak,, 4800RPM. The pipe had 3 steps arranged 1/2" apart so that at 4800RPM the first step would be "in Phase" just after the intake valve opened,, the second step (a deeper step) was timed to TDC overlap,, the third step was timed just before the exhaust valve closed. 3 low pressure Pluses in the same intake stroke. No other pipe combo came close. This pipe also affected the carburetor too, big time, but that's outdated info now.
In closing, now maybe you can make an educated guess as to which pipe is the best.
I sincerely hope so, WP