Hey Westsnip
You want to run a 2" pipe over to the new shower and install a 2" p-trap directly underneath the new shower strainer assembly you will be installing in the new shower. The vent for this shower needs to connect into (and roll above centerline of) this drain pipe within 6 feet of the new p-trap.
You can probably connect into the old vent...will probably find it as you go along.
Let me know if you have any more questions and also what type of custom shower you are considering installing....a few guys online here, including me, have installed tons of them, so get back to us if you want help with that.
For now, I am sending this site to you...check it out..great info. and pics.!!
How to Build a Shower Pan
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