How much fuel pressure would my '98 TF 3.0Ltr produce?
(as in PSI)

Fuel pressure is a function of load.

At idle, with the vacuum disconnected to the regulator, you should have around the 35-38psi mark.

Remembering that the fuel pressure regulator is the unit which sets pressure. It is a diaphram and spring valve, designed to inhibit the flow of fuel to the return line. In doing so, the pressure is raised enough to lift the diaphram and open the valve. A spring holds the valve down so the force of the fuel pressure has to overcome the spring to release. The spring pressure is 35-38psi. Vacuum acts on the spring side of the diaphram to lift the diaphram at vacuum and low load, reducing fuel pressure.