How much to rebuild a 65 200 6cyl and C4?

From that statement, it sounds like he's just recommending that you get lower ratio gears in the rear end. Steeper gears can be better for acceleration and quick launches. However, at highway speeds your engine will turn higher RPM's (e.g. w/ 3.00 gears you may cruise ~1K RPM, but closer to 2K w/ steeper gears).

If you don't own it yet, I would simply drive it when you get it. See what you think about it and make your own decision. EDIT: It may simply be a matter of personal preference based on how you intend to use the car--regular driver vs. racing vs. weekend cruiser vs. ?.

If you decide to go w/ new gears, I would have them installed professionally since you state that you don't know much about the mechanical side. I would say gears and install would run you in the neighborhood of $200-300--depends on what prices are like in your area.