Available in '64 and '65 with Slant-6 or V8 engine; '66 with V8 only. Conversion requires the transmission, bellhousing, shifter with bezel and boot (Hurst in '64-'65, Inland in '66), and floor pan section to cut/weld in. An overdrive version of the 4-speed was offered with the Slant-6 in '75-'80; it's not a difficult swap if you get all the right pieces, which is harder than ever these days.
Sounds like the car presently has a pushbutton automatic. Those are cool/fun and work well; unless your wife is absolutely rabid to shift gears, there are probably better places to spend upgrade money.