Mcmaster lists 1" NPT OD at 1.315" at the threads. It would seem that 1-3/8" hole would be ideal (1.375"), I don't think you're going to get there with a 1-3/8" hole saw. They walk, creep, whatever you want to call it and the hole will be bigger than the spec of the saw. I'd go with 1-1/4" and file it out to the right size. I'd also recommend doing a test hole in a similar material before doing the real deal.