Value of Stock vs. Modified to Increase Driveability

I will answer the question with the understanding you are asking value in the sense of money value. At one time I had a stock 69 Barracuda 318 2 bbl. I have made changes to meet my interests and love the car as I made it. I believe in a short amount of time you will realize that a stock 318 car really isn't much interest to people in relation to your question of "value". The "stock" value is found in a more rare car.
On the other hand, how you "value" the thought of it being stock is a personal choice. Some people cringe at any change to stock.