I actually you liquid gasket remover to dissolve the crap on pistons like that. Let it sit for 5 minutes, scrub with a brass brush. and repeat.
Your heads are at the limit. .005 is the max you can shave off those heads and stay within spec. If you would prefer to get a replacement head, shoot me a PM. I have a pair. One head has a couple of bent valves, but they only have 20k mi on them and are not warped. The only thing I would want from your heads is the cams since I bought these heads strictly to have the cams reground.