I test fit my 15's on my 14 bolt with discs and just a few casting marks on the calipers needed to be knocked down.  I hit it with a flap disc and then they fit primo.  Also, are you 100% sure that your 12 bolt and 14 bolt are gonna have the exact same spring perches?  The 14 bolt is a monsterously huge and heavy axle, I don't know what kind of lift your gonna be running but the pumpkin on the 14 is massive compared to the 12.  Just some things to think about.

yea i know the pumpkin is huge on the 14 bolt.... suspention wise on my truck me and my friend are goin to link it with air shox in the rear and im making a front leaf pack that will equal out about 5ins of lift and ima be on 35s till i can save up and get some decent 37s