If the rear-end is stock 1954 it would be a 3.90 and switching to a 3.55 would drop your cruising rpm only a little less than 300 rpms.
It's a good idea to crunch your own specific numbers when planning upgrades for your vehicle. Bear in mind that the stock 235 has a torque curve that starts at about 1600 rpms and effectively tops out at about 2500 rpms with 2100-2300 giving you the optimum efficiency. After that you're just making additional noise and sucking the life out of your 4 main bearing engine.
Try plugging your numbers into this calculator:
http://csgnetwork.com/multirpmcalc.htmlIn reality, you'd have to drop down to a 3.08 with the 1:1 4th gear to give you 65mph at 2400+ rpms with a 28" tall rear tire. If you really want to use your truck on the open road you need to consider retiring the sm420 and upgrading to something that makes a safe 300 mile over-the-road trip a fun experience.
Dave
