{{{ fuel ine size }}}

well the OP did buy a new Carter Hi-Volume mechanical pump. sooooo I'm betting it already has 3/8ths in/out.
if not who cares? but a couple $1 adapters from home depot.

take out the old sender and add a 3/8th line. cost: some solder, and a piece of 3/8ths tubing,
and a little time.