I'm confused... so even though it runs fine, doesn't smoke... you don't trust it to drive 100 miles without a problem, but are considering a different motors that's been rebuilt without any real way of knowning if it was rebuilt properly?
I'd echo everyone here... step away from the internet/motor/anyone trying to sell you anything and enjoy the car for at least a few weeks/months. After that, do some proper research and learn about what you already have. Learn about how to do basic maintenance to refresh the motor. Do you know how to adjust the carbs? Do you have the proper tools? Is anything leaking or worn out that needs to be replaced? Once you get some knowledge and some time behind the wheel... you'll be in a much better position to decide if you need more power and how you want that power made (up high, down low... engine swap/rebuild/ect).
