My 91 gmc 5.7 pickup is running lean, almost like its flooding,do any body have any ideas

How long since over-all tune-up? Any applicable trouble codes? You may need fresh fuel? If your check engine lite still works, you should be able to check some codes, yourself? I don't know liter size of engine?
Is the vehicle flooding? Use a gage and check proper fuel pressure. Do you have tbi fuel injection, fuel injector/s in the throttle body. If so, the fuel pressure is 9--13 psi. Also, the coolant temp sensor could cause it to flood. Could be other issues?

Have you had any issues with your oil level raising? You could have an injector sticking open slightly letting the residual fuel pressure that is in the line bleed fuel into the engine after shutting the engine off.

Leaking fuel injector?

you changed the spark wires but did you change the plugs..and if its a carburetor check the distributer as well

Clearly some fancy diagnosis and testing is needed and checking the fuel pressure will be part of those checks as would testing the injector spray patterns.
If the injectors are producing droplets that are large or are dribbling or leaking the oxygen sensor will see the result as a lean mixture and increase the injector duty cycle making the situation intolerable at idle when the intake air speed is too low to break up large fuel droplets.

Just an idea...

how are the tune-up components,if they need replacing this can happen as the extra fuel when cold puts more stress on weak components.(plugs,wires,cap,rotor,air filter,etc)it could also be carbon build up on the back of the intake valves,this can cause the carbon to absorb fuel(like a sponge)until either the car warms up or the carbon is saturated.this can cause a lean condition when cold.the way to correct this is to have a top end fuel service were a chemical is run through the motor to break up the carbon and clean the intake surfaces.

could be running rich. The fuel pressure regulator is screwed into the throttle body. Take the screws out and pull the cover off the regulator. The diaphram should be right there. Take it out and inspect for tears or holes. If any problems, replace the diaphram. The regulator DOES pass fuel back to the tank if the pressure delivered exceeds the design pressure of the regulator. If the regulator is failing low, that could happen, but I don't know how it could fail low. Please let me know if you have more questions.

check the catalitic converterif ti is red hot after 15 minutes of running its clogged

Coils ever get replaced? They create all the spark, if not enough spark no combustion so it will act flooded.

did you take out the thermostat?
If not do that if so it might be a feul mixture problem. To lean will make it run hot