looking at buying a 1985 F100 running a 302 Windsor crate motor with a 3 speed automatic gearbox.
It is a dual cab with a tray back.

So my question is can I upgrade the 3 speed gearbox to a 5 speed gearbox?

And how hard/expensive would it be to get done?

if its not possible how bad is the old 3 speed with fuel?

any help would be appreciated.