Ponton, Trim

Hello, I purchased a '59 220S that had the wrong engine and trans in it (mid-70's model). I found an engine with Type Plate M180.III 180.924Z and rebuilt it. I believe the transmission that I used was out of a 190SL, I don't have the number with me. The engine very quickly revs to high RPMs and seems as though the gearing is incorrect. For example, the car seems most comfortable if I am in third gear by 25 mph. I was told that all Ponton transmissions are essentially interchangeable with minor gearing differences, but now I am wondering if I have a problem with this marriage.