Several options are available to you. I would try to find a SM420 transmission, which is the tranny used in trucks from 1947 to early 70's. This tranny is synchronized and will be much quieter. You will have to shorten the first driveshaft to make this work.
Is the rear end a two speed? There are several different ratios available for two speed rears--just make sure you don't get one with a low speed (high numerically) ratio. I had a 42 with a two speed Timken rear end that would go 50-60 comfortably with a full pressure 235.
If swapping to a two speed diffy is not an option, you might be able to find a "Brownie Box" which is an auxiliary transmission that would have underdrive, direct drive, and overdrive. There would be some fabrication involved including driveshafts and aux tranny mounts.
You may also consider a NV4500 or some other new transmsion with overdrive. I am not sure what type of bell housing these take so I can offer the least amount of info on this because I have not personally done this swap.
