I want to install a 3/8 fuel line from my fuel tank to the stock fuel pump and a 5/16 return line back to the tank. Where up front do I plumb the 5/16 return line into the fuel system. And where in the tank does the return line plug into? I have a '69 383 that had a stock 5/16 fuel line setup but I want to install the larger 3/8's line with the 5/16 return line for future horse power growth. Can I order off the shelf lines for a '69 Plymouth B-body 440 or Hemi? I know I need a 3/8 sending unit as well. Any advise is much appreciated.

You would need a 3/8 sender with the return. You would have to modify it to the 5/16 return. Order one 3/8 line and one 5/16 line. Order a 440 fuel seperator and make a line to go 5/16 to 1/4. That should about do it. Why do you want a 5/16 return?

here's another option.
fuel return setup

I just ordered a complete 38ths factory bent set from inline tube today, they even have the clips and hoses. Whole kit was under 100 bucks.