The outlet fitting may need modifying. Wes does not modify the outlet fitting. He doesn't know what kind of supply pump you have. The supply pump should have a pretty even hum. I agree with you that it sounds as if the supply pump is not getting constant positive feed from the in-tank boost pump.
Check:
1. The in-tank boost pump is operating normally . . . i.e. the pickup screen is not blocked and the short connector hose from the boost pump to the outlet flange is in good shape and not leaking. With everything back in place, deadhead test the boost pump by connecting a short fuel line from the tank outlet flange to a gauge. With power on the pump, you should get 3 psi.
2. Put the system back together completely and turn the key switch on and listen for the supply pump. Go to the gas tank and remove the filler cap and listen to the inside of the tank closely. You should hear some dribbling from inside the tank. That's fuel from the return line going back into the tank. Although not common, the return pipe inside the fuel tank has been known to varnish up and stop the return line flow (commonly in cars that were stored for long periods of time with fuel still in the tank). The result of this is that the supply pump, which runs constantly, does not get enough fuel flowing through it to keep the pump and fuel cool, in addition to creating very high pressure in the feed line to the injection pump (very dangerous since the internal pressure relief valves in the L-jet supply pumps is over 50 psi). The fuel heats up in the supply pump and eventually creates a vapor-lock. The pressure then drops in the line, the low pressure light comes on, and the injection pump gets starved for fuel.
Now, if everything checks out . . . . . fuel filters all clean, in-tank boost pump providing a constant 3psi head of pressure to the supply pump, no kinks or internal defects in the rubber fuel lines, and the electrical connections to the pumps are all clean and tight and providing steady 12v power, then I would go ahead and narrow the restrictor, per the instructions in the guide.
I installed an L-jet supply pump about 4 years ago in my 74 Spider. Under engine load, I'd get an intermittent low pressure warning light with the restrictor unmodified. Once I narrowed the restrictor to 1/16" the pressure stayed at a nice 13-15psi regardless of engine fuel demand.
The problem with the L-jet supply pumps in a Spica application is that the delivery capacity in gallons per minute may be somewhat less than the OEM pump, hence the need to narrow the restrictor to keep the pressure above 7 psi.
That said, I believe the specs of various Bosch pumps is different, even though they may look identical on the outside. I don't know exactly what spec Centerline is stocking for a Spica application.
All of this sound real complicated, but it's really quite simple. These Spica cars are getting old now and most have had almost no preventative maintenance done to them. Things like rubber fuel line deteriorate from the INSIDE, not the outside and seldom get replaced until they're leaking, even though they've been sluffing off rubber bits internally for quite a while. Again, like the short little piece of connecting hose on the in-tank boost pump. If my car was equipped with a in-tank boost pump, you can bet that I'd replace that 10 cent piece of hose with a new one, just so I didn't get left stranded because I was too lazy to do a little PM on a 30 year old piece of machinery.