cleaning up the slant six head

Instead of the 50% Muriatic (Hydrochloric) acid solution, try vinegar straight on a sacrificial part. It's a little slower, but I think that you'll like the overall results better.
I've quit using Muriatic entirely after trying vinegar. Safer, easier on the environment, easier to dispose of, and doesn't have caustic fumes to worry about breathing and rusting what ever they land on (BT,DT too many times).