an fittings

Usually one size step is all that you get in an AN adapter. -5AN (5/16") is virtually non-existent, so finding an adapter to it will be difficult. The shortest route that I can see to 5/16" hose from a -8 fitting is two adapters, one -8 x NPT, and one NPT x 5/16"

Don't know what the power from the engine will require, but stepping up to a -6 (3/8") fuel supply isn't a bad idea. Depending on what you're doing you may be able to use the existing 5/16 supply line for a return.