When it is time to sell.

Pick one and focus on it and truly make it yours. After having a 51 Willys 4x4 pu that was a true pain to get parts for and I was always having to adjust the points and carb.

I decided that the 65 Barracuda was going with EFI and HEI. If the car isn't very rare the more modern you can stand to make it the more time you can drive it.