[LazyPup]

If you have a municipal water supply your main water shutoff valve shold be located at the water meter, if you have a well, the main is on the house side of the well pump pressure tank.

The final position of the trap is critical so I would begin by selecting the new tub to insure I had the exact rough in dimensions for the drain.

Before you seclect your new tub, measure the lenght overall (stud to stud) of the opening. (You could measure the tub enclosure wall to wall and add one inch to allow for sheetrock.)

Tubs are commonly available in 60" or 66" but can be special ordered in 54" overall.

I would definitely remove the old tub first to give more access area.

The two existing galvanized iron pipes can be easily cut with a reciprocating saw fitted with a metal cutting blade. (here is a tip, reciprocating saws wear out blades quickly. You will find that spending the extra money to buy a quality blade such as a Lenox or Dewalt is well worth the investment as they will outlast the cheap generics 3 to 1.)