I don't know the most "pro" answer, but since no pro has responded, I'll take a shot.
Whenever I come across a galvanized stub-out, I like to screw on a FIP (female pipe threads) PVC adapter using teflon tape. In your case, you'll need the adapter to reduce from 2 in to 1.5 in. Next, I cement on a standard trap adapter to the spigot (SP) end of the adapter and now you are ready to install your 1.5 in PVC trap.
This is what a trap adapter looks like: