The best 3 speed gearbox swap is to fit an FMX. Do you have a Fordamatic or a 2 speed auto. If you have a Fordo it is possible to fit an FMX as it will almost bolt straight up to your bellhousing with some minor mods. These days probably the most popular is the AOD as it provides an overdrive, though it is not cheap when you factor in the adaptor plate. The 2 speeds fitted to 59's, I thought, we're fitted to 332 and 352.
This illustration should let you know what younhave.
Regards