I have an original 99 WS6 18,000 miles and and looks new inside and out. What performance modifications if any can be made without lowering the value of the car.
I think any mods other than swapping on a different set of wheels and tires would hurt the value in the long run.
I have a 2002 WS6 convertible with 29K on the clock and it's still factory stock with no mods. As time goes on you'll see that cars that were left alone with low miles will become sought after and command the highest prices.
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1964 Tempest Coupe LS3/4L70E/3.42
1964 Le Mans Convertible 421 HO/TH350/2.56
2002 WS6 Convertible LS1/4L60E/3.23