5/16" Fuel Pump Inlet for 340?

For weekend fun and cruises, it seems like plenty of carb IMO. I would not worry over the 5/16" in and out of that pump at 400 HP; the main pressure loss in the 3/8" tank line will be under 1/2 psi at that fuel flow rate and the 5/16" fittings won't add much to that. The 5/16" sender and pickup would be more of a concern.

BUT your main issue is that the Carter M6866 pump is rated at 25 GPH and you will exceed that at 400 HP, so IMO your main concern is the pump itself, not the fitting sizes. Time for an HV pump at least; you don't want to burn a piston top at peak HP due to leaning out.

http://carter.opticatonline.com/part/carter-m6866-mechanical-fuel-pump

PS: And thanks for asking this question....I got to thinking after replying and asked my son, and we realized had just thrown a stock pump on our new 340 with the same HP rating... Whooops! LOL