906 head spring seat machining

Stick them on and go. It's fine. If you chose to have the heads done properly in the future you can look at itthen. I wouldn't care about .040 less isntalled height. It's got another .200" before coil bind, and we're probably talking about less than 30 pounds more on the seat and over the nose. I'd be much more concerned if the stem heights differed by .040". If they're good - run it.