FWIIW, the gray smoke is troublesome. This sounds more like excessive oil in the combustion chamber. The results of a compression test can help narrow this down.
All black smoke is more likely to be an excessive rich fuel mixture.
The fuel pressure measurements taken with the engine off are not much value. To confirm the correct fuel pressure with an external gauge, pull and PLUG the vacuum reference line. The gauge should read around 40 PSI for the Mustang. Rev the motor and verify the Fuel pressure stays around 40 PSI.
NEVER drive the car with the vaccuum reference line disconnected. This is just for a test to make the external gauge read the same as normal air.
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.