Resale value isn't everything. Life is short. If you're "battling yourself" every day because you want to modify the car but worry that it won't sell for top dollar down the road, you have to decide what's more important to you. Did you buy the car to build it up and enjoy it or was it solely a financial investment?
I got my car completely stock and had family, friends, and everyone else tell me not to touch a single bolt on the car because it was in such great shape and would be worth more stock. But I do not enjoy a car through leaving it sit in the garage collecting dust, never getting to turn a wrench on it. I got this car to have fun. I never plan on getting rid of it so what do I care if my modifications make it worth less. These days the same people who told me to never mod the car are always impressed when I install something new, asking how fast the car is now, etc.
It is true that many people on the market (including myself back when I was looking for mine) will not want a modified car because they don't know who did the work or if it was done well. Other people will just want to know who did the work and how long ago it was done and it won't stop them from buying the car.
If you want to mod the car, mod the car. If you want to keep it for resale value, keep it for value. If you want to mod the car and still sell it for good money later, do quality work, keep records of who/when/where did the work, keep old/stock parts safely boxed up and organized in case you want to switch back or if the next owner wants the old parts, and you should still get a pretty penny for it. As these cars get harder to find, people will have to be less picky about whether or not it's 100% stock. Beggars can't be choosers. If someone wants your car they will buy it.
As evident in that eBay listing, even a car with modifications will sell if it's in good shape with quality work and a record of what's been done to the car.
