The original Thunderbird prop from the 40's was an experimental thin blade design by Hamilton Standard towards the end of the war. I've read that only two were made, and one ended up on Thunderbird with the other on Beguine. It was rumored that Jimmy Stewart became scared of the blades after seeing how much they flexed during a ground run. Beguine's was lost to her accident, but Thunderbird's was eventually replaced by the more common square paddle blade Mustang prop. The experimental prop survived into the late-70's when it ended up on the Whittington Brother's first Precious Metal P-51D until it was damaged after a run in with a car on the ramp at Reno.
Yes, you are correct. That would be the correct prop for Excalibur, Paul Mantz's other P-51C Race 46, Jackie Cochran's Race 13, and a few Cleveland P-51D racers as well. Interestingly enough, the more serious racers at Reno have found that the cuffed blade is the faster of the two props. The late paddle blade prop showed up on Mustangs towards the end of the war, which is why you see them quite a bit on post-war and Korean P-51D's. The props were interchangeable, so plenty of airframes were still flying with the cuffed blades at that time. As always with modeling, it really depends on the reference photos of your subject aircraft.