Advance Adapters will be your best bet. With the Suburban being a 4x4, you'll have to deal with adapting the transfer case to whatever transmission you end up using. You'll need both the correct adapter case and output shaft to connect them.
Another option would be to find an early '80s GM 4x4 truck with the MY6 4 speed, which has an overdrive 4th gear. You'll want to find one with the NP205 transfer case, which will be the same as what you have.
The issue you're having now, though, is not the transmission, it is the rearend gearing. Higher rearend gearing would be one way to fix it (3.07, 3.73, and 4.10 were the common options), as would an overdrive transmission. Both will change the final drive ratio.
