I have an 88 F-150 4x4, 6 cyl 4 speed, low geared as hell- at 65 sounds like its ready for take-off, was told I could put a 5 speed in, and get better mileage, and lower rpm on engine -- ( save $$$ on the liquid gold)

have found a 5 speed out of a 91 4x4 -- what other parts from the 91 am I gonna need ( driveshaft- ect) -- any input would be nice before I spend the $$$$$$$$$ on the trans.

thanks