Without seeing the car in person, I think that you did alright on the price.
The Boss 302's will hold their value better than a GT. And there could be a potential for an upside... In 40 or 50 years. Have you seen what the average price for a 69-70 Boss 302 is nowadays? Lol. And a Boss 429 is through the roof!!!
Just take good care of the car (Like you seem to already be doing!), and most of all, drive it and enjoy it! That's how you will get the true value out of it.