Tub/shower remodel

This is my first post. I'm updating my combo tub/shower with new cast iron tub (30X60)and new travertine walls. Can you widen the tub width to 34X60 by building up the back wall? I'm thinking of creating a back ledge by adding two 2X6X60 along the back wall and securing them to the existing studs before adding back boards. I hope you understand my question.

 

A conventional tub has a raised flange on the two ends and across the back. This makes the installation very waterproof. Are you planning some kind of deck-mounted tub? There are more difficult to make waterproof.

Are you aware how much a cast iron tub weighs? There are quality alternatives such as high end acrylics, and the cast resin models. Cast iron is still probably considered the Cadillac.