A picture might help.
I installed a new Square D box in my 1986. I did not like the breaker used in the original. They would connect but there wasn't much hold power to keep them seated.
Something like this model: Model # HOM612L100SCP, it is a main lug panel.
I used a six breaker box. One breaker is used for the main breaker and it is wired as a back feed to the panel. The result is you have a main disconnecting breaker (30 Amp).
If I remember correctly I have four other 20 Amp breakers, one of which is dedicated to the AC unit.