I'm converting a 'carb' car to EFI (Toyota&filter) and need to add the fuel return line to the tank. Every one I have seen has entered from the top of the tank (along with the fuel pump line & fuel qty wiring). This may be an 'ease of manufacture' feature I suppose.

Any reason I cannot enter the tank below the fuel level?
I have a fitting that would be convenient to use, but it sits at about the 1/4 mark on the tank, which is tall & narrow, and about 10 gallons capacity.

I've been confronted with a similar situation on an old truck and I chose to put a return line on the filler pipe.  The filler pipe can be easily removed from the vehicle, drilled, and a long tube run inside the filler tube that dumps down closer to the central part of the tank brazed in.  Note that many older filler neck pipes were lead-coated on the inside, so remove the coating before you do any welding on it (toxic fumes).  

Fuel tanks are designed to have as few entry/exit points as possible.  The main reasons are safety, and evaporative emissions, more so than any gains in mfg. efficiency.  It is safest to have no fittings or lines on the sides or bottom of the tank that could be sheared off or cracked, in a crash. I'm assuming you have no evaporative standard to meet. Besides, getting rid of a carburetor, will do FAR more to reduce overall vehicle evap. emissions, than any other single modification.  That, of course, is assuming it's installed correctly.

The only other concern I would have is if the vehicle is old enough to not have a carbon canister, and pressure could build in the fuel tank on a hot day due to fuel heating, would it be able to push fuel back up the line to the regulator.  Even if it did, would this cause any particular harm?  It entirely depends on your particular hardware and mechanization, but I doubt it.

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