[67Fbird]

already addressed above ...yeah the wide ratio would be more street friendly as the 2.20 first gear is a bit....slugish without putting some serious gear in the rear.
Moving on....so from automatic to clutch
Obvious parts? Z-bar and linkage stuff...(unless you hydraulic) New driveshaft YOKE (m21 uses a th400 yoke) BEWARE...of the length of NEW yoke as it should be the SAME length as your th350 yoke..there are heavy-duty truck models that are LONGER...which would make YOUR CURRENT driveshaft too long. You will need a different front u-joint accordingly. Don't forget the neutral safety switch and reverse light switch.
It should be pretty straight forward swap and with a little searching I am sure somebody has reference numbers as to WHERE TO CUT THE FLOOR. Also note WHICH flywheel you NEED? 2.75 or 2.5. Your's SHOULD be 2.75 but always worth checking BEFORE you go getting a flywheel.