You will need later bellhousing (48 up) to use a 55 and later transmission. I'm not sure what type side cover/linkage configuration a 46 has. The 55 transmission has a 4 bolt side cover. The 1/R shift fork has to be replaced on an AD truck to use the side cover on later trans. Yours may be similar. A 3 speed overdrive transmission would be a good option.
Fred
