I considered doing my own floor rebuild entirely with wood, and once I saw that the bus was supporting itself with no floor at all for an 8' section, I realized that it would have worked fine even with a 3" depth. 8" high wooden cross members (or even 2x6s) would give you tremendous strength. I'm not sure how that would go as far as an inspection was concerned, though. And wood is pretty damned expensive these days, too.
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