Toilets, Sinks, Showers, Tubs and Disposals - Installing a drop-in tub - need help!

I'm getting ready to install a drop-in tub and need to build a support frame. I've never built such a thing before and have some questions. For instance, what size wood is recommended? It looks lie 2x4s would be sturdy enough but too narrow to lay tile from the top edge of the bath to the top edge of the frame. How much space, if any, am I supposed to leave between the top of the frame and the tub - is the tub supposed to rest on the wood frame, or do I lay the tile then rest the tub on the tile? If it turns out the tub is not level when it's resting on the frame, do I put shims under the frame or under the tub, or under both? Any help and suggestions would be much appreciated. Thanks.

No pro, but I wanted to help ya out a little by showing you this article if it helps any.

http://www.doityourself.com/stry/buildbathenclosure

check back as I'm sure other members will post something.