When I did the rear frame rails on my '66 GT, the replacement rails (about 15 years ago) did not come through with holes (and sleeves) in them to bolt the 90 degree GT hanger brackets to the frame rails. As a result, I positioned the new brackets and welded them with my wire-fed welder to the rails. Get the exhaust system where you want it, then clamp the brackets, remove the tail pipe / resonator and hit it with the welder. If I had caught that minor oversite when dismanteling the GT, I could of drilled, sleeved the rails to prevent crushing, and done it correctly. Next lifetime. Maybe a 90 degree drill could gain you access to drill, but I doubt it. Yeah, remove the gas tank too.