Buying a stock manual transmission with the corresponding rear axle, torque tube and pedal box could be expensive, on top of the purchase cost it would probably need a complete rebuild. Last but not least the conversion will probably decrease the value of the car, as the serial numbers for auto/manuals are different. My switch pitch conversion, which looked like a plug and play conversion, ended up as a 10k upgrade: even if the parts do fit, tuning and adjustment is labour intensive in this car, as there is so to say no room left in the transmission tunnel: even basic adjustments do require to remove the tranny, which in turn requires to move the rear axle / torque tube backward. So playing with unknown parts is probably going to make your mechanic either rich or mad... From my personal experience: a fully rollerized TH400 with a 1800 to 2400 converter is a good match for this car (9'' converters are too small). All the Teflon shims and seals should be replaced by steel ones as they do not sustain the heat and cause premature destruction of the tranny. If money is not an issue one can add to the list the super-light gear set sold by John Kilgore (and a few others). If you swap the tranny do not forget to keep the original modulator. I am not going to pretend that this would be as fun as a manual transmission, but with a 2200 converter the colombo is a new beast and the car remains "stock".