Then you will have to choose a SMALLER tub. You just said you don't want the footprint to increase. You'd barely have a deck with a 30 X 60 inch under mount tub. Five inches on either long side, if you want a "footprint" of 40 ". And an under mount tub requires SOLID SURFACE MATERIAL to mount. You can not use tile for that application. For that, you need a drop in tub.

When you remove the old tub, remove plywood on the top of the platform and install new plywood and make the cut out for your new tub leaving reveal for overhang.

The front is usually 4" without the step if you have a step you can have a larger ledge, as the sides and the back of the tub goes, it doesn't matter as long as its wide enough so you can clean it.

GN....

He wants an UNDERMOUNT. No plywood please.:)

Ohhh, in that case, what Jan said :-) Thank you!

I suppose it would be possible to properly detail an undermount tile tub deck, but I wouldn't recommend it. It's all people can do to find competent mechanics for vertical walls; this is in the advanced class.