89 gmc lean running condition

[Jordenw]

I have a 1989 GMC 1500 2wd, with A 4.3 L V6 from a 1994 Chevy 1500 4 x 4.
(The version with the counter balance shaft) and a five speed manual transmission. When you start the truck cold it will Idle up normal at first then makes like it wants to die then Back up again and it will do this for a few moments as it is warming up Then will eventually Idle normally. As I start out in first gear, unless I am extremely easy on the throttle it will bog down, Sputter, and act like it is out of fuel regardless of how much fuel is in the tank. Once it does go, if you push on the gas lightly it will accelerate seemingly normal but give it some throttle( 1/3 or more to the floor) it falls flat on its face till i let up just a smidge, it is very noticeable when it fades out. The only code that comes up is code 44 for lean exhaust. Now I literally blasted the parts cannon at this thing. New fuel pump x2(at first pump only, then just yesterday the entire assembly) fuel filter, complete teardown and rebuild of tbi unit including fuel pressure regulator and injectors, removed intake manifold and cleaned it out And reinstalled with new gaskets, new Coolant temperature sensor, new tps, replaced IAC, map, ESC and EGR with ones from a perfectly running truck, new EGR gasket (EGR was checked and vacuum tested as well) new distributor, plugs, wires, cap and rotor, new knock sensor, new o2 sensor and timing is set to factory spec of 0 degrees btdc with timing control unplugged.(also has new timing chain and gears) I have checked hoses for fuel leaks and vacuum leaks. All of which have made little to no difference. Am I missing something? I have tried everything I can think of. Any ideas?