There are some things that have been mentioned but got missed. You did not say that this transmission was ever in this truck. You were also told that it was a transmission from a Camaro. About the time the Camaro came out GM started using a larger front bearing in their transmissions, subsequently it has a larger bearing retainer. It is a fact that the larger bearing retainer will not fit in the hole in the early bellhousings.
The bearing retainer is about 1/4 thick and will not let the transmission seat all the way into the bellhousing. This could be your problem.
Fred
