I completely remodeled my bath room replacing everything. I now have 2 problems. First I'm replacing a slip on tub faucet with a front thread, and not sure how I should measure to cut the nipple to correct length before I sweat the new theaded end? Second the internal stopper seems to be seized, I can't move it at all. Thank you for any info any one could give me on this.

The copper pipe for the faucet isn't threaded in. so I can't replace it. And the stopper is lever type, I don't see a nut that holds it in place.