I'm running a 3/8" main fuel line on a 71 Cuda 383. It has a new mechanical fuel pump installed . What size is the inlet tube on the pump? If its 5/16", will the 3/8" rubber hose work without leakage?

If it has a seperate fitting get a screw in(npt) 3/8 hose barb fitting. If it is integral you can put a thin piece of neoprene on the barb then your 3/8 neoprene line over it but keep it thin as too thick negates the ability of the barb on the end to capture the hose and glue it so it stays in place as you slide the 3/8 neoprene hose onto it but I've done it w no problems just use 2 clamps & check them regularly. I have drilled out the stock 5/16" barb fittings on a stock "airtex" pump & soldered in a 3/8" fitting as the 5/16" at the pump is the same as 5/16" all the way through. Many people have had no fuel delivery problems w 5/16" lines but if your setup requires 3/8" I would consider another pump.