Another suggestion, most tubs I have seen are build around a framing that cradles the tub and supports it.
For example I have seen a 2 x 4 on the three sides nailed to the studs of the wall acting as a ledger if you will that the tub sides rests on.
Most tubs are built with a natural "fall" encorporated into the tub, but if not, you need to slightly slant the framing to make a one half bubble fall. Enough for the water to drain to the pipe. I would also make a frame on the floor, under the tub to cradle the actual bottom of the tub. Here I would use only wolmanized treated wood with no nail heads or protrusions showing that would eventually puncture the tub.
If this tub will have a shower spigot above for showering, I would use cementious board all around the tub. Even if you do not plan to have a shower, I would do it anyway, or at least up one foot all around the tub for splash ups.
I always think about all the stuff I should do when the thing is opened up and what I need to do so that I never have to go in there again.