Battery ground to aluminum head?

Aluminum is absolutely fine.
The power drops to your house are most likely aluminum.
Your starter is most likely mounted to and therefore grounded to an aluminum transmission case or bellhousing (and it also consists of an aluminum housing!).
Your distributor grounds through an aluminum housing.
Your coolant temp. sender may be grounding through an aluminum intake manifold.
Aluminum engine blocks and heads have been used for grounding for decades.
Now go have a beer.