I have a 500efi in my 35 fountain and it started fuel fouling plugs the other day my question is where would be the best place to start looking for the problem I was thinking fuel pressure or map sensor ?

Yep fuel pressure , then check fuel regulator / pressure, then scan the motor - look closely at MAP readings and coolant temp sensor readings. Next fuel injectors tested & flowed tested, then possible cleaned. I do that service as well for over 5 years now. Also do not forget about weak ignition components. Good luck.