The primary method that the Spica system uses to regulate fuel pressure is the outlet restrictor on the front fuel fitting on the injection pump. This small hole allows excess fuel to be routed back to the tank. The hole is sized to have about 15 psi on the higher pressure side of the system, even with a pump like the L-Jet Bosch pump. In the case of the Spica system pressure higher than 17 psi is completely unnecessary and in fact detrimental. The system is designed for between 10 and 17 psi.
That said, the secondary safety relief is different depending on the model year. Up through the 74 model year the front fuel filter had a Pressure Relief valve (set at 17psi) intergral in it. The 75s and later used a 3-port Bosch supply pump that had it's own PRV in it. Hence on the 75s and later, the front fuel filter did NOT have a PRV.
Hence, if you have a car that was fitted with a 3 port pump and substitute a 2 port pump, you have no safety PRV. Is that a problem? Well, it might be if you deadhead the pump somehow by pinching off a fuel line, causing the pressure to go up to the supply pump's capacity. Under normal operation with the outlet restrictor, the pressure should stay at 15 psi or so. Anytime you sub a different pump from the original (which is about ALL the time now since the originals are NLA), you should check the exact fuel pressure to be sure you're in spec. It may require you (as in the case with the L-Jet supply pump subs) to solder up the restrictor hole in the outlet fitting, and redrill it to 1/16th inch. This is because the capacity of the L-Jet pump is slightly less than the original OEM pumps, although still adequate the Spica system.
That said, I certainly recommend that there be a safety PRV in the system, even though most fuel hoses can take the pressure. That's as simple as getting a used front fuel filter (with an integral PRV), and fitting it to the car. It's a drop in replacement for the current one, with the exception that you'll need to cut the rubber return line and connect it up to the PRV ports on the fuel filter.
See the Fuel System guide you can download free from Ingram Enterprises, Inc. || Spica Pump Rebuilds
As far as suitable substitute pumps, some people have used the Master E2000 generic pump available in US auto parts stores. Otherwise, the L-Jet supply pump is a good subsitute as well. It's important however, that the pump have about a .6 gallon per minute unrestricted fuel capacity flow to provide adequate cooling to the injection pump (cooled by recirculated fuel).