Hooker:
Don't know alot about these kinda things..but I also have a 54 3100 with a 4 speed...have been on this forum for afew months and reading alot...I have been given the advise of switching the rearend gears to a 3:55 since my truck only goes about 48 on the highway. I would switch the rearend and give it a go before spending coin for another Tranny. Just might do the trick. I'm sure other folks who have done the switch will pipe in with their experience.
Would love to see some pics of your 54..have you posted any on photobucket? ( Free site ).
You can always PM me if any questions..since we have same truck and are trying to fix them up.
Thanks