67 Barracuda 273 Fuel line thread size???

No (as in none) A bodies were made originally with 3/8, and there were no return lines until "some" 70 and later. That is all "aftermarket". Depending on how much power you are going to run, you might consider a 3/8 Off the top of my head, I'd say if you are planning "much" over 400HP go 3/8