What is wrong with my 93 Silverado K1500 350ci, 5.7L engine?

I have a 1993 Chevy Silverado K1500 350ci, 5.7L engine. I haven't been able to get it to run since december. It has a brand new crate engine in it. It only has about 15k on it. All the things I've replaced since it started to act up. Throttle Position Sensor, MAP Sensor, EGR solenoid, Oxygen Sensor, Fuel Pump. It all started driving down the highway at 50 mph. Cruising along then it just shut down. Pulled over, vroom turned right over but was running as though it were on it's last drop of fuel. Inched my way to a gas station filled it up and it it ran absolutely terrible. Spit and sputter. First I replaced the map sensor and tps after reading my haynes manual. Ran just as bad. Then after I replaced the fuel pump it ran a tad better but not much. Took it to a guy he said my o2 reading was very low and never changed with throttle response. Changed that and it runs but still sputters before it shifts. It's really bad when you're trying to go up a hill. You can practicly hear it sputter in the TB like it were a carb not adjusted right. I've been dealing with this since december and I've stumped 3 mechanics so far. They just gave up and said idk what to do with your truck. My last resort is chevy but if someone could please just help that'd be awesome. Oh and now since the o2 sensor as soon as I hit 50 the truck is like a beast it goes up to 80 in no time flat. At high crusing speeds ypu would think nothing is wrong but as soon as you let out of the throttle you can just hear in the exhaust a different tone and a sput sput sputtering. Please help. Haha thanks