Hello everyone, I have trouble getting my '79 Mustang V6 2.8L/C4 auto to run properly. Here's the symptoms :
It starts up fine, idles fine, goes in all gears perfectly, doesn't smoke, but when I hit the road and start accelerating, it loses power, sometimes to the point of stalling, most of the times it will just be very slow and the maximum speed will be around 60mph...and consume a lot of fuel.
Again, all gears change smoothly, so smooth I can barely feel them.
Also, it tends to run out of fuel when there is still plenty in the tank.

I've done a LOT of work to the engine to attempt to solve the issues. The car ran perfectly before I pulled the engine and transmission to paint the engine bay. Here's what I've done :
- pulled engine and transmission in one piece, followed by a cleaning with a pressure washer (it was covered in a mix of undercoating, dirt and oil)
- nothing was removed from the engine, I didn't touch the kick down arm or carb or ignition, nothing moved.
- put it back a few months later
- replaced all the fluids
- replaced transmission filter, oil filter, fuel filter
- flushed the transmission fluid as much as I could and refilled with new Type F fluid
- replaced water pump (it was leaking at the shaft) and water hoses
- replaced fuel pump and hoses
- new spark plugs, wires, cap and rotor, coil
- rebuilt the carburetor (the symptoms were already there, didn't help much)
- checked the interior of the gas tank and sending unit, both in perfect condition
- checked the fuel line, perfect, not plugged

I do not know what to verify anymore. I was thinking maybe the vacuum modulator at the transmission could be bad, or the vacuum advance diaphragm as well.
What are your ideas ?

Thanks