OK. I think you have a few things wrong. DTC P0193 does not mean low fuel pressure. It does mean there is a wiring problem to the fuel pressure sensor (low VOLTAGE from the sensor).
Don't ignore this DTC!
Regarding DTC P0503 is directly related to the primary ignition low voltage coil driver. Since this is a V6, one coil driver run two cylinders. Which could certainly explain why it runs like crap. Check for bent/pushed pins to the coil pack.
This is why it;s important to post all of the symptoms and DTC's.
IMO, your problem(s) are wiring related. Either from mixing/matching non compatible wiring harnes.
The P0503 is causing the chronic misfire.
However, it will never run right until the fuel pressure sensor issue is resolved.
P0193 - Fuel Rail Pressure Sensor Circuit High Input (FRP)
FRP signal shorted to VREF or VPWR.
FRP signal open (gasoline only)
Low fuel pressure (NG only)
Damaged FRP sensor.
High fuel pressure (caused by damaged fuel pressure regulator) NG.
P0351 Through P0360 - Ignition Coil A through J Primary/ Secondary Circuit Malfunction
Open or short in Ignition START/RUN circuit
Open coil driver circuit in harness
Coil driver circuit shorted to ground
Coil driver circuit shorted to PWR
2003 GT Convertible (sold & missed)
2000 GT coupe (Craigslist project. Fixed. Now my DD). Windsor to Romeo swap.