I had a similar issue as others the motor would turn but the arms would not swing. I noticed that the transmission was connected to the motor arm with a rubbery bushing over the arm ****. None of the resto places listed this bushing and neither did the chain part places. It might come with the motor or trans but why spend so much for it? With much google-fu I saw Dorman had a bushing for the same years Chevy trucks and beyond. GM loves to use the same part on anything I got a 2 pack for about $6 PN49446. It did the trick and it no longer spins and binds up on the motor arm ball stud.
It's the white bushing. How this helps someone.