00chevyk2500

Hey my truck is a 2000 chevy k2500 about 5k ago i put in a brand new crate motor. All was running fine until a couple days ago i couldnt get it to start just turn over so i replaced the ignition coil what do you kno it started right up for about 1 day. try it again wont start i bought a new ingitiion coil again wont start i got ignition module still wont start i got a crank shaft positioner threw that in now itll start up and die unless u have the pedal to the floor and it wont go over 500rpm and missfires bad the whole truck shakes. Any ideas? Can not figure it out!