If yours has the original floor, your fuel line from the tank comes through a hole in the floor, under the console sealed with silicone then is attached to a fuel shut off valve mounted on top of a teak block, then the fuel line ran down the rigging trough on the starboard side, all the way back to the transom, where you can mount your water separator near the outlet in the trough. 3/8 ID was the standard fuel line size. This is the way the 17, 19-6 and 22-2 were rigged with the tank under the deck in those days.
Know what you mean,I was confused a little too. It’s 1/4 inch thread with a 3/8 barb.
Replace with 3/8 barb, also make sure there is no anti-siphon check ball in that fitting block. It will be a spring and a steel ball. If its there, remove it. It's required for inboard engines but not outboards. It can cause fuel starvation problems in outboards. No Teflon tape on those threads, fuel resistant Permatex which is what it looks like they used which is appropriate. Use that on the replacement fuel barb. Teflon tape is only good for water pipe threads.