The engine angle does not have much to do with it really. Other than how it might affect wind-up and twisting of the airframe. But you are right in that respect, that should not be a big consideration.
That is the only variable that I changed and received RPMs that were in the range that Jett suggests. I rotated the engine so the muffler was pointing down and got 19,000 + RPM with a 8x6. However, since the Wonder has no landing gear, I would have been forced to land on the muffler. It is strange to me, but that is the only thing that seemed to work. I'll locate a test stand and see what happens.
I may give the Viper a try.