I think you're definately on the right track (i.e. leaving the floor untouched). A couple notes - leave yourself more of a flange for the gasket. I don't think .100" will be enough given the iron block and aluminum heads. Thanks just move around a bit and I'm not sure how well that will stay sealed (or for how long...). I'd go another .025. Also watch your manifold fit. It better be perfect.