1965 Dodge Dart high RPM issue

Look for a ratio tag on one of the cover bolts. If it's still there it should solve the mystery (I'm betting 3.23). 185/80-13 is a really common size (cheap) and fits an early A pretty well.

A slant will run all day at 3000 RPM, and shouldn't be especially noisy doing it. Make sure your exhaust system isn't touching the unibody anywhere.