RE: Building a P82 with YS60's
What pipes are you using ? Pipes are often different. Even two from the same manufacturer can be different.
The "pipe system" length is more important than simply a measurment of the header length. If you have the high point of the pipe (or reflector baffles) at different distances, that will cause a different tuning point.
Easy fix for your observastion is to lengthen the pipe system for the "high" rpm engine an extra 1/4" and try it again. If they match closer, then clip the "low" engine a bit more to see if it will come up to match the 14,700 rpm side.
Bob