Like Markallen said check the length of the tailshaft(extension housing). IIRC there's 2 different lengths About 4-5 inches different.
Swapped tranny's in my 74 C10. All went well until I went to plug the driveshaft into the extension housing.
I think Chevy went to the same bellhousing pattern for the 6 and 8 in the early 60s. Pontiac/Buick can be bolted up with an adapter or by cutting part of the bellhousing off the trans