New vanity doesn't leave room for old J trap - how should I run the new pipe?
Adding a 45 to offset at the galvanized nipple and extending so a trap will fit  is the right thing to do. but do not raise the back side plumbing extend the tail of the sink. A straight shot down will flow better and have less bends to build up and create clogs. Even though I have all my threading gear I try to stay away from messing with old metal drains. There have been a couple of times on my own property I have started on a small job and ended up with all new lines to the septic tank. Putting in the 45 will take a little more room under the sink but it will be easier and less risk of opening a can of worms if the 40+ year old drain line crumbles. In my case it was a 1960’s home and even the cast main line was compromised. my guess that one of the prior owners had used a harsh drain cleaner regularly (this was back in late 90- early 2000 so that plumbing was only ~40 years old I haven’t seen new galvanized used since the early 70’s except in rare cases). So if you have metal it could be very fragile.