Buddy an original /6 Duster that was restored and turned into a 340 clone just sold for $62k at auction. If you do a quality mini-tub job on a car it's not gonna hurt the value unless it's a 1 of not very many and a full concourse restoration. On anything other than that it might actually help resale, especially if it goes with the build of the car- ie, a mini-tub on a pro-touring car isn't going to hurt resale one bit.