Hi Jimbo...
Really not so bad today...as long as using a shower pan made for this exact situation...
Check out the pic. below.
It is a pre-fabricated shower pan that replaces a tub exactly. You only need to move the tub drain slightly AND increase the 1.5" pipe to 2" (may even be 2" underground already).
Anyway, you would rip out the old tub and walls, align drain and then install the 60" x 32" pre-fab. shower pan in modified thinset mortar or similar bedding substrate, level the pan, then install 1/2" cement backerboard at walls, tile and should be all set!!!
This is far easier than installing a custom shower pan and as long as set in mortar base is just as sturdy and has less chance of leaking years from now.
Check out this link for information on building your own custom shower:
How to Build a Shower Pan
And then check out this link for information on installing hardibacker cement board properly:
James Hardie: HardieBacker 1/2'' backerboard
Obviously, I recommend the pre-fab. shower floor...
Let us know if you need more information...glad to help...MARK
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