Trying to squeeze the 3/8" rubber line down onto the 5/16" steel line will probably result in an air leak at the fuel pump inlet. If you warm up the end of a piece of 5/16" rubber line in very hot water, it will soften and stretch enough to slip over the 3/8" fitting on the fuel pump, and the clamp will just be a redundant extra precaution.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!