Backfires

I bought a 'the truck listed above k3500 dually and now it won't go over 2,000 rpm (at all), backfires and loses power if I push the gas to hard. I had the knock, cam and crank sensors replaced and the crank relearn can't be done since the truck can't hit 4,000 rpm. It sat for a good two or three years and was put back on the road in 2016 (at that time the truck acted like the rev limiter was set to 3,000 rpm and would backfire over that). The timing, rotor cap/ button, plugs/wires have been fixed/changed also. The truck also did not have catalytic converters either, someone told me back pressure could cause my issues also. It has been suggested that either it is my distributor itself or a fuel pressure issue, but I have no idea. This is so far above my pay grade and I'm absolutely lost. Any help at all would be greatly appreciated and thank you.