I would not recommend capping off a fuel line that is below fuel level if we are talking about the 68 in OP's avitar
If we are talking 68 fish, the sending unit/pick up is in front of tank half way down side, below fuel level not in top.
I was talking to OP, who just purchased a new tank for $180 instead of a $200 tank that has its feed and return already designed for pump and includes baffles which are very important for in tank pump or even external pump.Just seemed like trying to reinvent the wheel just to save a couple of bucks and it might not work very well ie no baffles. We are a fab shop and make many parts for swaps, headers, pulley systems, intake plenums etc but I would not waste time doing this as I did it in 1998 when I put injection in a 68 fish and it was the only way back then but now its literally $20-$40 more, delivered to your door with improvements. You wouldnt need baffles with rear mounted sump in a straight line car. Personally, I would NEVER build a car with pick up below fuel level as its cheap insurance against fire and leaking....I think those lock rings are a lame way to seal tank but its your car, happy fabbing