I am virtually certain that you will find that the hitch ball on your '70 Caravel
is 2 5/16". During the 1960s (1960-69) the Marvel 2" coupler was factory issue for all Airstreams according to my notes, but this doesn't mean that a trailer from the 1960s may not have been modified during the past 40 years. Beginning in 1969, my notes indicate that the switch was made to the 2 5/16" coupler - - I know that my '78 Minuet has its original 2 5/16" coupler which does seem rather large for such a small, light-weight trailer. (I can't verify the source of the original information from my notes as these were taken very early in my association with Vintage Airstreaming and I didn't do a very good job of documenting sources on some of those notes.)
You can find the hitch height recommendation at the Airstream corporate site via the following link:
Airstream Weights and Hitch Heights
According to the table posted ther, the height should be 19".
Good luck with your Caravel