I've got Mac Longtubes. You can use the OEM gaskets for them. Just make sure to get new ones. I wouldn't re-use your existing ones. Trust me, any maintenance on longtubes on a 4.6 is literally a pain in the butt, and having to go back a replace them later will be quadruple the work.
2000 Mustang GT-Former giggle gas addict
1990 Mustang GT w/ Vortech V3
***resident gun idiot***