there's fuel line reducers out there: plastic, brass, threaded, etx. so it's just figuring out what you've got and what you need.
for example here's some barbed brass ones:
LTWFITTING 1/4 in. x 3/8 in. I.D. Brass Hose Barb Reducer Splicer Fittings (10-Pack) HF3913R060410 - The Home Depot
but, depending on what you've got cooking it may be in your best interest and safety to just replace everything. that way you don't wind up with a cobbled together mess and potential for break down or spraying fuel all over a hot engine bay.
now let's see them pics!