Did you mark the frame for axle centerline before you tore it down? I didn't (just measured and screwed up) and wound up cutting out the IFS and moving it back 2". Not fun.

If you haven't, I'd strongly suggest putting the cab and fenders in place and marking (drilled hole so you can't grind it off) where you want the centerline to be.

http://www.chuckthetruck.homestead.com/page16.html