you could
solder a compression fitting onto your 5/16'' line and put the 3/8'' hose and hose clamp over it and nobody would know. it would all be hidden.
or
make the change at the fuel pump, one a 3/8'' and one a 5/16''.
or
as suggested buy a series of brass fittings and put that mess together and connect your hoses together. this seems like it may take three brass fittings and look wierd.
or
machine (if you have a lathe) a custom piece, or for that matter silver solder or braze a custom steel inline fitting.
gooday
jim


Gooday
Jim

small wheel moves by fire and rod,
big wheel fires by the grace of god,
everytime that wheel turns round,
bound to cover just a little more ground.