I have a 1957 3100. My 3 speed gave up third gear last year and I also wanted to cruise a little faster without running 6000 RPMs. I had already put a new ring/pinion in my rear (moved from 4.11 to 3.55 I believe)--that didn't make much difference unfortunately. What did make a huge difference is going to a Borg Warner T-5 with a 5th gear overdrive. I now cruise 70 mph at about 3200 RPMs.
There is a great article in an old copy of Classic Truck (don't have issue number for you) that I followed. I got a T-5 from a 1988 Camaro, and changed out the tail shaft with one from a T-5 from an S-10 so that the gear shifter location was just right for my bench seat. Toughest part was finding a clutch to work--article suggested a clutch for a Chevy Astro van that had a V-8 with a 5 speed transmission (there couldn't have been many of those). I went to Advance Auto Parts and asked for the above clutch and it worked perfectly. Best thing I ever did to help the driving ability of my old truck. To keep the truck looking original, I purchased a shifter handle from Lokar and it looks just like an old original 4 speed on the floor. Hope this helps.
