Postes on other forums but really stuck...

Hi all, I have just bought a 4.2 V8 Supercharged and it hasn't been out of limp mode since I got it.
All kinds of codes P0301 through P0307, P0347, P0003, P0116 list goes on.
New Plugs, Coils all ok, New Cam Sensor, New Throttle Body, checked Vacuums, New PCV regulator....
Slightly better but still when ANY load put on like slight incline goes into Reduced Power mode 😔

FRP on my reader says 320 kpa with ignition on. Once started that drops to 235 kpa and stays there at idle. Once moves fluctuates between 230 and 400 but absolutely zero power it's like it has an engine from an Austin Maestro! So bad. Revs but no power then back into limp mode. Sometimes no codes just OL-Fault on Fuel. Sometimes codes same as above despite all changes.

I'm thinking fuel filter but I think it in the tank? Could it be the fuel controller under the rear seat?

Please help, getting a lot of grief from the missus at the moment