From their manual:
"The return line from the injectors must be T’d into the return line of
the injection pump. Assemble the T-fitting with 2 HB-14516 and the
CP-1003 compression fitting/ PB-14316 bushing using thread tape.
Remove the rubber hoses attached to the Injector Pump Return line
and Injector Return lines. Reuse clamps. Install FL-1056 fuel line
from steel inj. return line to the T fitting, secure with HC-4001
clamp provided in kit and factory clamp. Install the rest of the fuel
line from the T fitting to the factory steel line at the frame. Cut Injector
Return line (measure twice, cut once), slip on compression
nut, ferrule; hold line tight to T fitting and tighten nut to 18 ft./lbs.
Secure hoses."
Just tee the return out of the ppump, and the injector return line. Then take that and put it into the 'R' port on the FASS.
Obviously the 'E' port goes to one of the two supply ports on the pump.
And the 'T' port goes to the fuel tank.