Ahh! E-bay motors.
The output shaft at the rear of the transmission is either short (usually for cars and short bed trucks, or long (for long bed trucks). Measure the output shaft housing on your transmission from the gasket surface to the end of the housing. It will be either 4", 6" or 12".
The TH350 is TurboHydromatic 350. TH400 as you can guess is the TurboHydromatic 400 and is generally found in trucks. These are both 3-speed automatics.
MT is manual transmission (three-speed).
If you have a mechanical cable (like a throttle cable, not electrical) running from the carburetor to the transmission, you most likely have the TH350. The TH400 had an electrical cable (for kickdown control into "passing gear").
You can use a multi-fit transmission. There are two bolt patters used to connect the transmission to the engine. Look at the top of the bell housing on the transmission. If you have a "valley" between the top two bolt holes, you have a Buick, Cadillac, Olds, Pontiac transmission. If you have a "peak", you have the Chevy bell housing. You probably have the Chevy bell housing (since the Monte Carlo is a Chevy product).
I would pull the transmission out to get all the numbers off it. Then you'll be sure to get the correct replacement.
