My 63 wagon with the factory 6-230 OHC engine will do 65-70 down the road with thefactory running gear. It has 4.27 gears and actually gets up and goes pretty well. Stopping is a whole other story, if you are going to drive freeway speeds look into a front axle disc brake conversion, or rebuilding the original to get proper operation, parts are available from several of the vendors who advertise on the site. My wagon has a stretched timing chain making timing the motor impossible and so it has some issues I hope to fix this winter, but still gets 19-20 MPG on the hiway.
You have to realize these are old and are going to have work needed to make them good hiway drivers, but that is part of the appeal, anyone can buy something to drive it takes a dedication to you vehicle to drive a Willys.