You will find that with most all transmission swaps you will have a faster first gear. Your 4 speed is about 7.14, or something close, in first gear. Only the NV4500 comes close to having as low a first gear and overdrive, at least for common transmissions that will fit. Although I don't think it is easy to swap to a 6 cylinder, very easy for a V8, call Advanced Adapters.
If you really need that low first gear then the gear vendors OD unit is probably the best answer.
An auxiliary transmission like a Spicer 5831 with the overdrive ratio is another solution, although it is a little bit heavy, needs two cross members of its own, and a good bit of fabrication to install it and the shift linkage. Unless you get lucky and find one still installed in a truck with all the goodies at a cheep price the Gear Vendors unit will be better bang for the buck. The Spicer unit will give you even lower gears, a direct, and an overdrive, assuming you find one that is not direct, low, and lower.
