Just swap the rear housing, the one that came out of your truck is a after market made by advanced adapter your truck came with a 700R4 transmission
You were 100% correct! After two different mounts that wouldn’t work I got frustrated and pulled the bolts off the rear housing and whacked it a few times with a rubber mallet. Problem solved! I couldn’t even see a seam and was convinced this adapter was part of the TH350 case. I just cleaned it up and bolted everything together. Fits like a glove.