I believe that Kaiser Willys sells the overdrive units in their catalog...If you have really steep gears (5.38s), this OD unit probably won't give you the highway speed that you are looking for...For what an OD unit costs, you can swap your ring and pinion gears at both ends to get the speeds you want, say 4.27s or 4.10s with money left over...New ODs cost around $1300.00 !! Remember, the Willys four bangers, L134, and the F134 are LONG stroke engines...They are built for low speed "grunt", not high rpms like modern fours...Keep rpms down so that your engine lives to a ripe old age...Remember too that a short wheelbase jeep gets "squirrelly" at highway speeds over 60mph...And you only have 9" drums...panic stops at speed can be very interesting !...Just keep the old girl in the slow lane...Been there..done that.