I've never been "comfortable" with heads that have been painted after cleaning. They COULD be (not necessarily) hiding something. We use WD-40 on them when they come out of the cleaner.
If you can get a "money back guarentee" if they're cracked or pitted beyond reason, go for it. 670 is a good cylinder head with even better "potential". Good performance head. Steps must be taken to keep static compression under 9.5:1 for 93 octane. "Dished" pistons is the most simplistic way, but we have had success with machining the chamber "larger" in a fashion similar to the later "open chamber" heads.
