If you are just looking to keep the rubber lines from dangling down, you could purchase some Adel clamps like these.
IMG_2038 by
Lawrence Alexander, on Flickr
You would have to drill some holes through the subframe and then find the right size self-taping bolts to hold them in place. Nothing wrong with using steel braded hose and AN fittings, but it will probably be bulkier and you might have clearance issues. Since the fuel pump is using vacuum to pull fuel from the tank and the return lines only have about 5 PSI pressure the braided line is really over kill.
Pontiac and Chevy engine Firebirds have the fuel pump on the driver side, so you don't have this problem with the supply line. I think they ran a steel line over the cross member for fuel return, but it was only a 1/4' line, so smaller diameter. I will see if I can find how Pontiac ran the fuel lines on the 403 Olds engine. It might take someone with a 1979 assembly manual.