I actually had my Muncie M-20 out and a transmission expert told me that the transmission needed a rebuild.I think the M-21 is a better transmission although im not sure what the differences are between the two.He gave me a riduculous price like 860$ to rebuild my M-20.Like I am some moron or something.I can get a rebuilt tranny M-21 for about 500$.With the 3:73's at 70mph you get 3200 rpm's??How tall are your tires in the back??Actually with the 4:11's its not that bad but I have no speedometer hooked up so I really can't say at 70mph with a 28 inch tall tire what RPM's I would be getting.Also I have never been on a highway before with my car.