The firstgensite shows 5/16" for 67-69's and it's the ID, it's under the parts illustrations,does not mean 400's didn't have 3/8", I had 5/16"ID on my 67, I replaced it with 3/8" ID. FYI Firebird had a 2 piece fuel line, one Front to rear and one on the cross member. You should never measure pipe and tubing to the outside always to the inside diameter because of the many styles and thicknesses of pipes and tubing. Sometimes tubing crushes on the end and measures smaller than it's actual size.
