A Warner T98 or T98A should be bolt on replacements, that is what I believe was used in the F-6 with M series, and also later Ford Trucks 53-56 or maybe thru to 64. They should have both large and small bellhousing patterns on them. After that Ford went to the T18, T19, and NP435 which probably will not fit the flathead. When swapping other transmissions in, especially with larger trucks think about the emergency brake as they were driveline brakes mounted on the back of the transmission.