Unless you nail the panel to a tree [Linked Image] whatever it is mounted to is a structure and the rules for grounding electrodes apply.

Even a simple 4"x4" post stuck in the ground with a panel mounted to it would be a "structure".

Here is the entire NEC defintion of structure.

Quote
Structure. That which is built or constructed.

It sounds to that the 6 AWG AL will be adequate for the proposed load.

If you do change to 6 AWG CU the largest standard breaker you can use is a 70.

If the AC unit has a minimum circuit ampacity rating of say 25 amps that still leaves 25 amps for the compressor.

I am guessing that someone sized the wire to the AC based on the breaker not the load, this is good for you. [Linked Image]

Bob Badger
Construction & Maintenance Electrician
Massachusetts