There are always more than two ways getting from point A to B. There is a synchronized transmission that is a bolt in operation for your truck, the f-6 model trucks that are equiped with the 6-cylinder (M-series) engine used the 4-speed synchro-silent transmission as standard euipment. It is also optional equipment in the F-4,5,6 with the R-series 8-cylinder and H-series 6-cylinder. This is straight out of the 1949-51 Ford F-series Trucks Shop Manual( with the 52 supplement). This is also a very good reference. My next suggestion would be to find the 5-seed overdrive transmission that was standard in the F-7 series, there is also a non-overdrive that was standard in the F-8 and optional in the F-7, however, I can not tell from the diagrams and it doesn't state it explicitaly that these transmissions have synchros, they don't appear to. I don't seem to find the overdrive ratio listed anywhere in the text, maybe someone out there knows? The only problem is insuring that there would not be any clearance problems, the cabs on the big trucks may set higher of the frames than the f-1,2,3. Just some suggestions.