Minor surface damage won't be a problem. If it is warped then resurfacing is the usual fix, the barrel surface should be checked and also resurfaced as required. Minimum material should be removed to reach flatness. Hold down fixturing is tricky for the head, it can be bent by holddowns and sping back to warped. The barrel section is much stiffer and less trouble.
Aluminum is not friendly to grinding. I've never seen a head or barrels done on a surface grinder. Maybe there are wheels and procedures to do it? The surfaces I have gotten from machine shops after milling were really nice and I don't think a ground surface would improve sealing.