REDUCER FOR FUEL LINE TO CARB
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 a brass one:
HOSE BARB BRASS UNION 3/8" X 5/16" (HB-BRUN-00C-00A)
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!
edit: you say 1/2" thick, so i'm guessing that you're measuring the outside of the fuel line. it's likely that your lines are 5/16 or 3/8" ID