Powder-coating could be problematic: the resultant coating is insulating so you'd need to scrape the coating away anywhere there's a need for a bolt-through earth tag.
I'd get it bead-blasted as a first approach, then perhaps an acid-pickle (chromate) to clean out any deep-seated pitting.
Then a very-fine misting with etch-primer [you still want to be able to see the underlying metal through the misting]. These guys:
http://www.arc-rite.co.uk/aluminium-...y-sizes-1564-p are worth consulting.
Then spray the topside of the chassis with whatever topcoat colour floats your boat, but leave the underside unpainted for better contact where you need to bolt-down tags/valve-bases etc.