I'm not real familiar with the older (than 48) trucks.
Does yours have a bracket on the transmission for the brake and clutch pedals?
To use most any later or different transmission I expect you'll need a 48-53 bellhousing which will have a more useful bolt pattern, still fits your stock mounts, yet won't have provisions for brake and clutch pedal. This thread and pictures linked in it is one option for replacing the brake and clutch on the new bellhousing.
https://www.stovebolt.com/ubbthread...in=157950&Number=1109074#Post1109074After you have that sorted out you have a few options.
Later model 4 speed with syncromesh is the SM420 (found in 1948-67 trucks), and it bolts in to the later bellhousing, no adapters or anything confusing at all. This is still a direct drive transmission, will go just as slow as you already do.
Some chance you can find (and fit) an old Clark overdrive 5 speed from something like a GMC 400 series. they were made with the standard GM bolt pattern, but not all of them. The overdirve models will have a VO after the model number, for example 205VO. The "O" indicates overdrive. I'm not sure what troubles you may run into but possibly the input shaft is a little extra long, requires a spacer or machine work to input shaft. Will likely need a different clutch disc with larger splines. Might need 5-1/8" bore in bellhousing or take care of that with the spacer/adapter or machine work to bearing retainer or bellhousing. I have not done this swap, but I am sure one way or another it can be done.
Downside is finding the transmission and finding one that is any good, they're now quite old and parts are scarce.
A more modern but not quite cheap option is an NV4500 5 speed overdrive. This requires a spacer/adapter like so
http://www.advanceadapters.com/prod...ansmission-to-a-gm-v8-adapter-plate-kit/Also need to enlarge the bore in bellhousing from 4-11/16" to 5-1/8", or ask if they have a more suitable adapter already made to suit.
Another 5 speed is the T5 from an S-10 pickup. It is a pretty easy swap but not really suitable for a full size truck. I'd save that option for the 1/2 ton guys.
Next you could use a separate auxiliary transmission like the Spicer 5831 for example. This takes more fabrication, adds another shifter, more weight, and some complexity when shifting the unsynchronized second transmission. Also takes some effort and or money to find one.
There are also overdrive boxes you can bolt between engine and transmission like this "Ranger torque splitter", I've heard good things about them.
http://www.advanceadapters.com/prod...overdrive-27-with-a-4686-retainer-index/Then you have bolt on overdrives behind the transmission, very expensive.
https://www.gearvendors.com/cg2wdt.htmlUS Gear had a similar product, now out of production.
I'm sure there are other options but that covers many of them.
Grigg
