Original down pipes were double wall. surprised it didn't rattle. The original exhaust manifolds will develop tiny cracks where the castings come together.
Another option for phillips head screws that I have had luck with on airplanes and others where trying to avoid drilling out. A small dab of valve grinding compound in the screw head and a hand screw driver with as much pressure as you can apply and maybe a wrench on the screw driver if you can. This has saved drilling out atleast 75% of nasty bad screws for me.
I can't believe nobody's mentioned the " shake & break" yet ?
This tool fits into a standard air chipper/hammer (not included - use the one you already own). The vibrations loosen painted, corroded, rusted, dirty and overly tight screws and fasteners. While you gently turn the screw using the attached handle. Because you are not forcing the screw loose, you don't damage the head. So you can re-use the fastener after you remove it. I'm tellin' ya, this sucka really works! It's fast. It's easy. And can literally save you HOURS of cursing and throwing tools across the shop...(not that I've ever done this)...