Before you hit the fuel or turbo, check the throttle linkage and make sure that full throttle means the IP commands full fuel. Mis-adjusted linkage or even a bent pedal can cause the IP to never go wide open.
Even with the C-turbo you should get at least 7 psi of boost! My (brand new) D makes 15 plus if I accidentally let it. Ideally, you would have an exhaust gas temperature gauge installed and then bypass the FDC. If that does not give you the 7 PSI boost (procedure is in the TM) then it is time to turn up the fuel a bit more.
There are different schools of thought - one is to turn it up and drive consciously, staying out of danger EGT and boost territory but enjoying the power when you need it. The other is to follow the TM and adjust to a level where sustained pedal to the metal won't produce excessive EGT.
Your truck, your choice!