Buying a used one you don't know if it will work. I have a bunch of 120's I put in a couple of them to fix my bouncing 150 needle. They all bounced too.
I took it to Redline and they refurbish my 150 and all is cool.
If jbc426 doesn't actually use the trip odometer, I don't see a lot of downfall to jbc426 using a 120mph core speedo to covert to a 150mph.
I mean you are going to spend the money refacing and refurbishing a used 150mph anyways. Why not save the core 150mph speedo core.