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1977 F150 transmission swap

New truck new questions.
I’m now driving a rebuilt 1977 F150 with the 390. The question being, am I able to switch the 4 speed out with a 5 speed? If so, what would be the best route to go? I’m ok with losing a bit of power in order to increase my fuel economy.
Thanks for the help

You might compare the mating surfaces, lengths, output shaft splines, speedometer drive and input shaft/pilot bearing length and diameter. It might be much simpler and cheaper to swap for a taller geared chunk.

All a 390 needs is a 3-speed…Putting a 5-speed behind it is loves labor lost…You will not notice any improvement in fuel economy…5-speeds were never offered in 352/360/390 pick-up trucks…So installing one now might be quite a challenge, a very expensive challenge…

You might investigate what your rear axle gear ratio is. It’s stamped on a tag attached to the third-member or back cover. If it’s 3.73, 3.92, 4.11 then there might be room for improvement…If it’s 3.23, 3.52 just leave things alone…