99 Vortec 454 Timing Issues

[72k20Farmtruck]

I've got a vortec 454 out of a 99 suburban we swapped into a 1982 6.2 diesel truck. We used Howell Engineering's harness and ecm. Changed injectors a few weeks ago because of hot injector leak, typical issue. Been driving the truck a lot and we've been happy with it so far until earlier today I was at a stop light towing about 8,000 pounds when I started to take off the truck sputtered and died. Tried cranking and cranking and it keeps backfiring through the exhaust. Acted like the timing went fully retarded. Anyways we got er back to our shop and I changed the coil because that's the only ignition piece unchanged and still backfiring through the exhaust. This time it backfired so loud I went deaf for a good few seconds. Ears still ringing. Here's a list of what we've done previously:
Spark plugs
Rotor
Cap
Ignition Control Module
Cam Sensor
Crank Sensor
Coil
Injectors

Motor had about 140,000 miles on it before the swap so I can't imagine the timing chain jumped. All help is appreciated I would like to make this truck work again it's been a really fun project and a great work truck. Thanks!