Sputters at about 40 tachometer starts going crazy

Sierra 3500 had motor and transmission replaced 2 years ago. Ran great then fuel pump went out, replaced it, the cap, the rotor, plugs plug, wires, coil. Now it idles great but once you drive about 10 miles it starts sputtering backfiring kinda exhaust smells awful. But then you let it sit a minute and it’ll run fine again. The gauges when the key is off mark everything as if it’s on when key is on all are at mark they should be when key is off. We have spark, we fuel. So what could be causing this? We live about 50 miles from the nearest town so it’s not like we can run in to the parts store or junk yard real quick. Please if anyone has had this problem before or knows how to fix it let me know. This truck was a Cab and Chassis a utility bed was added later 5.7 vortex 1 ton 3500 4 wheel drive.