Mike,
Two details to consider.

The generally accepted rule for our models is to size your exhaust about 1.4 times larger than the intake. It's supported by people like Andy Lennon who wrote the popular book, "R/C Model Aircraft Design" and by lots of experienced modelers who know it works.

Also, there is a group of modelers who've proven intake and exhaust sizing for a few engine sizes. They're the guys who've been flying R/C Pylon. The event demands about the same engine performance you're probably wanting. Pictures of their airplanes ought to be fairly easy to find. Those ought to give you an idea what intake size can work at their speeds with racing engines running nitro fuel.

Good luck.