If those numbers are what the market is bearing for fair shape convertibles, then my recent purchase was a steal. I'd drop the bottom and top on "fair" by a couple grand.
Here's where it gets tough. You will spend as much or more to restore a Sprint as a 350 or 400, and it will be a thoroughly enjoyable and unique car. But should you decide to sell, you will not see the same increase in price due to your improvements as you will on a 400 car. If you plan to keep and enjoy the car, then this difference makes no difference.
Sprints are a lot of fun, especially when coupled to anything other than the two speed auto.
Vikki 1969 Goldenrod Yellow / black 400 convertible numbers matching