Keeping a 1967 beetle at original value

Unless it's a really immaculate and perfect original car, I don't think that changing the seat belts will hurt the value. Beetles aren't really that valuable anyway. As long as you keep the original parts in storage, any part that you can change by bolting them on and not cutting up any original parts are fine. When you sell the car, you can put the original parts back or offer them to the buyer. The only modifications which will ruin the car's value are things which can't be easily reversed, like cutting a hole in the dash for a modern radio. Changing wheels or even engines is fine, as it can be undone.

Bolt on changes can be easily reversed so don't worry about it.