In high school, my best friend and I gutted his '66 Mustang, cut out the floorboards (rust) and then welded in new metal.
Used about 2 gallons of roofing cement to seal the cabin before replacing the interior. That think was tight....didn't really do it for sound, but thinking back it probably improved the sound dramatically.
The good ole days!!!

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Bryan Harter
1992 500E Smoke Silver
2002 CLK55 AMG Cab Silver/Black
2001 Harley Fatboy - Pearl White
2004 F150 FX4 Silver/Grey
1969 GTO Liberty Blue
1956 F100 Black