Decreased Value Using Aftermarket Performance Engine Parts on Stock Truck?

[learyw]

I have a 1978 K20. Its almost entirely stock and in great shape (after I replaced a few rusted out panels). It will be repainted this summer. It is not a show truck and it's not a frame off restoration, but I have it fixed up pretty nice. So, my questions is:

I want to get some more performance out of it. Its almost completely stock with numbers matching (except my carb which is as close to original as I could find). Will the truck lose value if I add aftermarket headers? Aftermarket cylinder heads, manifolds etc? What about a spray in bed liner?

I dont have any intentions of selling it, but I would hate to do something and ruin the value. They are quickly becoming more in demand and its value has really shot up in the last few years. Thanks. -Will