Re: kind of foundation for cast iron bathtub
You didn't clearly state what type of tub this was: a freestanding (clawfoot) tub, an alcove-mount tub, or a deck-mount tub. I'm guessing it's alcove-mount.
(An alcove-mount tub is the "typical" bathtub with a false front and three tiled walls going straight up from the tub's rim. A deck-mount sits in a hole on a raised platform, with the deck of the platform extending outwards on all four sides from under the tub rim.)
The aforementioned advice is good, but seems to be oriented toward freestanding tubs.
If you have an alcove-mount tub, many of them are designed so the rim rests on ledger boards attached to the wall. This implies that the walls -- and the flooring underneath the wall -- must be substantial enough to support the weight of the tub, water, and bather.
If you have a deck-mount tub, of course you'll want to make sure the floor framing is sufficient. Some plumbers will pour concrete on the subfloor and set the tub into the wet concrete in order to provide full support for the tub to prevent flexing (unlikely with cast iron!) or the deck sagging. I think this is more common for fiberglass tubs than cast iron.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.