Jeff3600, I have a 1954 3600 currently down to the frame. I plan to use it to tow a car trailer. I am using a 1954 261 with a the higher compression 235 head (848). Transmission is a NP 540 5 speed (from a 1958 2 ton)this should solve the problem the sm420 has with the wide ratio range between 3rd to 4th which will make towing in the hills more difficult. Currently in the process of installing a Dana 60 with 3.73 gears setting pinion angle, drive shaft length and perch mounts. The Dana is from 72 Ford F250 and 3/4" wider than the stock 1954 rear axle. Measured from wheel mounting surface WHS. Except for the shock and perch mounts it is an easy swap. The rest of the truck will be original and unless you have a trained eye the engine and trans will look original also.