Hi Matt, and welcome!!
If the P30 has a Dana 70HD or Dana 80 rear the GVW as a P30 was approximately 14,000 lb.
Your plan sounds reasonable and achievable to me..
It should, as the axle swap you describe is what I have done with my truck. I even have plans for a full air ride rear suspension, similar to a Hendrickson model HAS-120 or the Kelderman kit offered for a F550
Check out the picture link at the bottom of my signature, the axle swap folder in particular.
The short answers:
The rear is a simple swap, only need to move the rear spring pads out approximately 1" each and at the same time adjust the angle to suit the drive-line, this assuming you use leaf springs. If air ride to start with, then the P30 rear axle width works well with the 36" wide rear frame, the rest is up to you.
Front axle will need springs with 2" more lift, and the big thing is it needs to be
narrowed approximately 10". Chances are you can use the original steering gear box if you please.
One thing to consider is the 19.5 wheels, even with 8R19.5 tires at about 33" tall I think they look small and out of proportion on a 1.5 or 2 ton truck. There is a 22.5 option, but somewhat difficult to find the wheels. Check out this tech tip:
http://www.stovebolt.com/techtips/wheels/Sounds like fun, and a lot of work,
Grigg
