Vibration ONLY at 75 mph
most of the paint is original lol..
my guess is a loose body panel or part that vibrating at 75mph due to wind effects. when you are driving what area does the noise come from?
i know you said you can feel it everywhere... but is one place stronger than the rest? or could your passengers tell you where they thought it was coming from?
when you are coasting, the wind is still blowing. have you ever been driving with a headwind and have the vibration at a different speed? even one time?
also when coasting, the driveshaft and parts inside the trans are still moving. that's why you never coast down a mountain with the trans in 1st or 2nd gear even with the pedal pushed in, the clutch is still spinning. (people have exploded clutches that way, they are not made to spin 15k rpms lol)
i know you said you have changed everything. what things in the drive train have NOT been changed? it sounds like you changed every single drivetrain part.
it will be either a loose body panel or one of those parts in the drive train that have not been changed.