T&T: No inlet for fuel return?

[Bob Clinkenbeard]

On my new to me boat, the builder and PO did not plumb the fuel system 
correctly.  The fuel return line ran to the Racor fuel filter and caused the 
engines lift pump not to be able to work properly...so he added an electric 
pump in the fuel line to compensate????  Oh well, now I need to do it 
correctly and the fuel tanks only have a fill inlet, a vent outlet and a 
pick-up tube outlet.  There is also a fuel gauge on top of the 40 gallon 
tank.
My question is. how can I return the fuel to one of the tanks without making 
a new inlet?
Any way to "T" the vent fitting for this purpose?
Does anyone know the amount of fuel that is returned by an M-35 Perkins 
Perama circa 1993?
Thanks,

Bob Clinkenbeard
24' custom trailer trawler
http://home.bellsouth.net/p/PWP-aboardbobbinalong