1999 V6 Rangers do have the FPR in the gas tank on fuel pump assembly, its a Returnless fuel system
The 4cyl 2.5l still used the FPR on the engine with return line
In 1998-2000 V6 Rangers the FPR is on the fuel pumps output hose, not a good idea
Ford change that in 2001 by adding a 3 port fuel filter
Not sure if adding an inline FPR will work, outside of the tank
And I haven't seen this type of FPR sold separately
1998-2000 V6 assembly seen here:
https://www.partsgeek.com/assets/products/fulln/JKRV41F6.jpg
The round part above the pump is the FPR, just a spring loaded valve set for 60psi(appox.)
Fuel pumps are capable of 80+psi
FPR has the white hose on the back that sends extra pressure/fuel back to bottom of the tank
Picture here of a used 1998-2000 setup:
https://www.ranger-forums.com/howtos/FuelPump/FuelPump019.jpg
If your system pressure is too high, above 65psi FPR is stuck open
If pressure is too low, under 45psi then could be weak pump or failed FPR
Ford changed this system in 2001 because fuel pressure at the rail/injectors was jumping around too much
They change to a "semi-return" system, FPR still in the tank but hooked to 3rd port in fuel filter this stabilized pressure at the rail
Newer system has no connection between pump and FPR:
https://m.media-amazon.com/images/I/615iOy3BtXL.jpg
Returnless systems got a Damper on the fuel rail to prevent the build up of pressure waves in the fuel rail, its just a rubber diaphragm that absorbs pressure waves so they can't reflect back off the end of the rail and build up
Vacuum hose is just a safety requirement in case it leaks
Return systems FPR on the rail did the same thing, its rubber diaphragm absorbs pressure waves but also regulates pressure, it's vacuum hose stabilized pressure using engine load/vacuum level