Turning walkin shower stall into bathtub/shower combo

[Brian_Houston]

Question:
Current shower drain (2" pipe) runs 5 feet, under slab, to vent stack (5" cast iron vent pipe) of toilet. Will need to move the drain 3ft to wall for new tub.
After moving the drain, the complete length of the pipe will still be around 5 feet.
We will have to reduce the drain line at the trap to 1 1/2 in order to connect the tub drain/overflow. Do I need to add another vent?
Or is the main stack sufficient?

I've uploaded chicken scratch sketch of drains. Please advise.

[hj]

As long as you install a trap at the end of the pipe, rather than the elbow you show, then it will work, even though it might be a bit longer than code would like it to be, depending on which one you are using.