I had this turnkey engine installed in my '74 back in mid 2015:
https://www.summitracing.com/parts/nal-19210009I had it done by my local GM dealer because I am woefully unequipped for an engine swap…..and I wanted the warranty. I know I overpaid, but that's the way life was at the moment.
The biggest problem they ran into was the A/C compressor location. The original was on the right side of the engine bay, and the new is on the left. They had some custom A/C hoses made up to get it all back together.
It also turned out that the bolt patterns were different on the flexplates, so I wound up needing a new torque converter.