You can go to the hardware store and get some clear plastic tubing, and that can be used as a sight glass-type material. BUT, I don't know if your fitting are JIC (37 degree flare) of if they are ORFS style (O-Ring Face Seal).

ORFS style, might be a little harder for you to find fittings to screw into that hardware store clear plastic hose (and probably use hose clamps on the outside, too)

I'm pretty sure a NAPA/ JD / IHC store will have JIC fittings. My guess is that you would purchase 3/8" ID clear hose and get the thick wall stuff, if it is available. For this job, I would worry about pressure, because you will probably mount supply clear tubing, before transfer pump = suction; then probably mount return tubing after fuel return check valve = very little resistance going back to fuel tank.

  Edit: On last photo, that engine is using #6 JIC fitting, for both return and supply (inlet) = should be able to add clear plastic tubing to those fittings.

Special note: Fuel needs to travel "Uphill" in any type of sight glass, to get an accurate view = this forces any possible air bubbles(some air bubbles are created just during the installation of sight glasses) thru the system

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clear tubing.jpg (5KB - 789 downloads)

sightglass.jpg (3KB - 791 downloads)

fuel system sight glasses.jpg (75KB - 800 downloads)

fuel_pump_3116 fuel routing_in paint program.jpg (72KB - 865 downloads)