whole lot of goodies here if you're looking to convert to a 4 speed.
https://www.kijiji.ca/v-transmission-drive-train/strathcona-county/m21-4-speed-transmission-with-chev-alum-bellhousing/1306617656?enableSearchNavigationFlag=true
Any reason this wouldn't fit just as well in a 1967 B body?
1966 Strato Chief 2 door, 427 4 speed, 45,000 original miles
1966 Grande Parisienne, 396 1 of 23 factory air cars
M21 is close ratio . need wide for B body . jmo