Max RPM for da slant?

225 or 170? Piston speed at a given RPM would be significantly different between the two, although I believe the piston weight is similar. That said, I would think 5000 would be a practical limit on a 225 if you expected any sort of reasonable street life. Honestly, port size will probably limit breathing at or before this point. These engines were never designed to be high RPM, high flowing deep breathers.

I'm not sure if there are any dimensional stability/resonance issues with the cranks, but below 5000 these shouldn't be too big of an issue.