I haven't attempted such a swap, but suspect it could be done. Some parts swapping and perhaps a bit of minor fabrication may be required. The older 904's had mechanical speedometer drives. I think the transmission mounts may be in different locations. No idea how the computer would react to the transmission swap.

Some people buy a manual or video and rebuild the transmission themselves. The 3 speed transmission are rumored to be easier to rebuild than the overdrives.

With the 3 speed transmission, you may want to consider changing the rear axle ratio if you plan to drive it on the highway.

Personally, if it was my truck and the money was available, I'd follow Gene's recommendation!