The M20 is the 4 speed used in early '60s GM performance cars. It's more or less a beefed up T-10. GM based it on the T-10, beefing it up for use behind the higher power engine, mainly the 421/428 Pontiacs and the 409s in the SS Impalas.
There shouldn't be any issues with putting it in, but like Grigg says, there's really no advantage to it. A T-5 would be a better choice. Better ratios and an overdrive.
