Engine appears to be a 1966 427/390hp, originally installed in a Corvette. Transmission is from a 1969 Chevelle, coded as an M20 originally. But the input shaft and cluster gear could easily have been swapped to make it an M21.
I'm not sure if this the right spot but I thought you all might be interested in the seeing the original paint markings and the rear end code & date on the gears.. These pics are from an early 67 basket case I bought in May that I have been putting together.