I'm almost certain it will work depending on your bellhouse!
There are 3 different flywheels. Very early CJ3a's and MB's, CJ2a's have a 97-Tooth flywheel which takes the 10-tooth starter, these mount differently in the bellhousing (also different) and the metal plate between the engine and flywheel is different to, they will not match.
And then there are the 124 or 129-tooth which both take the 9-tooth starter. The 9 tooth starter is the same in both 3A and 3B, available in 6 volt, 12 volt or 24 volt. The Cj3a and Cj3b both have the same bellhouse and the "closed nose" type starter can be exchanged as far as i am aware.