I have a feeling it's quite obvious but why exactly is a fuel press gauge important? Just so you can monitor fuel pressure? Sorry for the stupid question but I've always used fuel press gauges to help with tuning my gasser and I have it plumbed in directly to the carb at the inlets after the pump.
Properly mounted and monitored, a fuel-pressure gauge is the indicator of the condition of the lift-pump and filter.
Actually TWO are even better than one; the second gauge need not be in the cab, just screwed into the system where it can be read from under the hood.
ONE gauge, plumbed between the filter and injection-pump, will tell you when there is filter blockage or a weak lift-pump.
TWO gauges, one EACH SIDE of the filter, will tell you if the problem is the filter or something ahead of the filter.
That second gauge can be bought for about the cost of a filter and the first time it keeps you from guessing wrong and replacing a good filter, it has payed for itself.
In my truck, the normal fuel-pressure with the piston-lift-pump is 15- to 16-PSI.
A quick burst of power might pull it down to 10-PSI for a second.
When I notice the normal operating pressure gradually losing ground, I know that most likely the filter is beginning to plug with debris.
When it drops to a constant 10-PSI or thereabouts, I know that it is time for a new filter.
If the engine shuts down, or is starving for fuel, the dash gauge tells me that pressure is way down; a comparison with the pre-filter gauge will tell me if the problem is the filter or something before the filter, such as line blockage or a break in a line. (it is NEVER the lift-pump when that lift-pump is a piston-pump
)
Let's say the wire to the fuel-solenoid has a bad connection (providing the solenoid hasn't been done away with and a KILL-CABLE installed), and this bad connection shuts the fuel off, thereby shutting off the engine.
You crank and crank and no start.
The know-it-all bystanders will aver that you should replace the filter; you hitch a ride to town, buy a new filter, put it on, and still no start; now they will blame the injection-pump itself and have your belly all in knots over nothing.
As soon as the engine died, a few rounds of cranking would have brought the fuel-pressure gauge up to normal operating status and you would have instantly known that there was no problem there and to look elsewhere.
I have been running mechanical gauges in my rigs since '83 and have installed MANY mechanical gauges in engine rooms where just the paint job -Awlgrip =$250+ a gallon-- on both engines that operate at 75 psi, and marine gears that run 200 psi +....so long as common sense is used and not relying on cheap hose/fittings...
