Tim: Your Caddy tailfin comment is interesting, since the P-38 was the actual inspiration for the '48 Cadillac fins, the originals.
Harley Earl and his merry band of designers went out to Selfridge Field, which is northeast of Detroit, when the airplane was still in its very early stages. One of the guys that went with him was Bill Mitchell, who succeeded "Misterearl", as he was called, and I got to know Bill in the '80s through work. Anyway, they saw the twin booms and all, and played with various design iterations throughout the War. And, the fins ended up on the '48. Take a close look at the fake vertical airscoop on the side of a '48 Cad Sixty Special, and you also will see the intakes on the Lightning's twin booms. Another example of (car) art imitating (airplane) life.
The fins on the model here, at least the first ones shown, remind me of the '61 Caddy. Of course, things really got out of hand on the '59s, the icon for the entire decade, probably. This was done by Dave Holls, a Mitchell protege, and a great guy that I also had the privilege of knowing though my interest in classic cars.
So, if you want a "Constellation" type fin, look to the '48 through '53 Cads. For the more rakish, larger sizes, later in the '50s.
Actually, on this model here, since we are playing around with cardboards, try a conservative, rounded Connie pair. If if looks out of place, just tear it off!