5.3 & 4L60E Help

Hey everyone. I?m a new member. Hoping to gain some knowledge on this forum & help out when I can as well. I have a 1957 GMC that has been swapped with a 2002 5.3 & 4L60E. The guy that did the swapped crashed my truck in his shop, which bent the driver door backwards & damaged the fender, closed his shop, moved my truck to his buddy?s house. In the end I had to basically go remove it from the guys house without it being completed. The whole experience was a nightmare. It?s now up on Jack stand with what I believe is a rear main seal leak, the transmission is shifting hard & seems to be slipping & it?s throwing like 10 codes on the OBD. I believe some codes should have prob been deleted. I?m hoping you guys can help me with determining what codes need to be handled & which ones can be deleted from the computer. I?m in So Cal so if anyone has a good tuner that could get me straightened out that would be helpful. And maybe you guys can look at a few photos of my oil leak & tell me if you think it?s rear main.not sure how to add photos yet. I?ll work on it.
Thanks in Advance.

Welcome to the forum, and sorry about the nightmare with your swap. You?re in the right place for guidance.

With a 5.3/4L60E, oil leaks often come from the oil pan gasket or rear main seal cover, not the seal itself. Clean the area, run the engine, and take clear photos to trace the leak.

Hard shifts and slipping are often tune-related (shift pressures, solenoids, wiring). Scan all OBD-II codes and post them?some need attention (MAF, TPS, transmission codes), while others (EVAP, rear O2s, EGR) can be tuned out.

Next, gather your codes and leak photos. A good SoCal tuner like Westech Performance can delete unnecessary codes and adjust transmission tables, making a big difference.