I've got an L83 (with mods) and 8L90E from a 2016 GMC Sierra, installed in a 72 El Camino. After install it would crank and run for a couple of seconds and the cut off. Took it to a tuner who was able to get the engine running, but could not do a tune because the trans will shift from 1st to 2nd and then 3rd, but then drop to neutral. He says he has looked at everything computer related and it is commanding the proper shifts, but the trans is not doing them. He does not do transmission work, but believes it is a mechanical issue. Has anyone seen anything like this? If not I'd appreciate any troubleshooting suggestions, before I take this to a transmission shop.