77 midget gear ratio

Anyone know an easy way to change the gear ratio for a 77 midget 1500. It has a 4 sd tranny and only does about 45 mph in 4th gear at about 3k rpm. Not much good except for running around town. Any help would be greatly appreciated.

Would it be as easy as changing the rear differential. I've seen a few on e-bay for sale. One was a 3.90 and another a 4.22. I am assuming that the 3.90 would be the higher ratio. Is this correct? and is there an easy way to tell which one I have in mine? Also my transmission is noisy in first gear, but only in first. Anyone else have any experience with this and what it took to fix. Any help would be greatly appreciated.