But the CIC jagmaster I have, would any 25.5" jazzmaster neck fit on there? Anyone know?
I'm not sure if the Jazzmaster neck would FIT in the neck pocket. I do know that even if it fits, you will have severe intonation problems and the works of that due to your Jagmaster being a 24" scale and a Jazzmaster being a 25.5" scale. I suppose it could be accomplished by moving the bridge and such to accommodate the 25.5" scale, but I'd imagine you'd be filling in the old bridge holes and everything and it would end up being a giant pain in the ass.
My recommendation would be to sell or trade your Jagmaster and try to pick up on of these...
Squier J Mascis Jazzmaster - $399.99
You may still be wanting to change the pickups in this, although this does get very good reviews saying it's a terrific guitar.
I know this is not an easy suggestion to accomplish and you may like your Jagmaster to the point of not willing to part with it.
It is just the route I would go if you were after a Jazzmaster neck and the J Mascis sound. Hope it helps a bit, and hopefully some other
people can chip in even more about different routes you could take.