If it's not really about tuning or wiring. Then it belongs in here.
On the initial build I just sprayed it down with wd40 occasionally. This time it’s in the car, oil still in the sump, etc... best way to prevent rust while the head is off for a little while?
Do things actually rust in SoCal?
I don't think WD40 would care whether the engine was in the car or not, and I would not be afraid to use it. You will not be using enough to worry about oil contamination, but if you were concerned, you could change the oil after the initial startup with the new head.
They do rust just not as quickly. Depends on the material and how prone to rusting it is. I have no idea what the cylinder walls are made of but I can tell it’s aluminum.
Personally I've just soaked multiple layers of paper towels in fresh oil that you intend to use and stick that to the walls. Never had any issues.
That’s a great idea!
Glad to be of some help
Something which might work out of the kitchen is Glad Press'n Seal. It's a product which physically attaches to whatever it comes in contact with after a little pressure is applied. It is (for the most part) water proof, so will keep most (if not all) of the moisture out of and off of the bare metal. You'd just have to ensure the deck surface is completely clean of all oil (I'd leave oil in the cylinders), then have at it. Just don't remove it until you're ready to go. (Note: I have no affiliation with Glad, but do enjoy their products when needed :o)