1965 Dodge Dart high RPM issue

Your original rear gear ratio was 3.23. As said before, make sure you are actually in 3rd at 45, sounds like you are in 2nd. As for changed gears, it would take 4.88 or 5.13 gears to get your revs that high in 3rd, neither of which are available for the 7 1/4 as far as I know.