[crash]

Mark, sounds like you have a can of worms there. Main case assembled 1968 model year put in a Chevelle, with a 1963 1/2-1965 707 side cover with a 429 tail housing 1964-mid 65 Chevys and also Pontiac A-body's thru the 1970 model year.
But all these parts will work together, as long as the gears and sycro are in good shape.
The number of rings on the input can usually tell the gear ratio...Hope this helps...Crash