A 3/16" compression fitting works perfect for the injector return line.
Less than a $2.00 at true value.
Mine has been on my AirDog for almost 2 years with no problems.
As far as the stock fuel filter goes,
I ran my supply line to the injection pump, I did away with the lift pump and pre-heater. A chevy fuel blockoff plate works perfect.
The return out of the overflow, I cut off and left 2" of steel line and used a 5/16" x 3/8"npt compression fitting,to a 3/8" coupling then a 3/8" x 5/16" hose barb to a piece of 5/16" hose.
I then ran a small piece of 5/16" hose to where the injector return line is and used a 3/8" threaded tee with 2-3/8npt x 5/16 hose barbs on the run of the tee, out of the bull of the tee, I used a 3/8 npt x 5/16" hose barb with a small piece of 5/16" hose.
That connects to 1/4"npt x 5/16" hose barb to a 1/4" coupling then to a 1/4"npt x 3/16" compression fitting connected to the factory steel injector return line. Out of the run of the tee, it goes back to the fuel tank, where I used a bulkhead fitting to tie the return line into. All the compression fitting and the 3/8" npt tee are brass.
I did away with the sending unit connections. My fuel line comes out the bottom of the tank and my AirDog return is on the top of the tank and so is my overflow return line. I hav 2 seperate returns on the top of the tank.
I posted some blurry pics.
Been like this for about 2 years with no leaks or no air leaks.