3 speed would like lower rpm's hwy?

Is this car an automatic or manual trans? Had the same problem with my Valiant. The car was a pain to cruise at highway speed (poor slant was screaming) but I liked the dig it had due to the gears. Since it was a three speed manual, I swapped in an OD 4 speed. Now I have the best of both worlds. I have the same gearing I had before with an overdrive 4th that gives me sane cruising revs. All I had to swap was the bell and trans, front driveshaft yoke, and cut a hole in the floor for the shifter. It's a little more complicated if you have an automatic though. Basically your options are a 500/518 swap which is not a straight bolt in, or a Gearvendors overdrive unit. Kiesler has an OD auto swap kit but it uses a Generic Motors transmission. Don't know about you, but the thought of GM parts in my Mopar makes me want to:puke: