Scott its not really that easy, but the stock axle center line is 7.391 inches behind the two holes in the top of the frame above the trailing edge of the front crossmember.
The reason I say its not that easy is because you are probably changing the ride height as you do this, and the correct visual location of the axle centerline changes as the truck comes down.
Most people feel the rear axle is too far forward to begin with. The front axle looks right at the stock height, but as it comes down the visual center moves forward. I would encourage you to determine your desired ride height, then figure out where you want the wheels to fit in the opening, and set the new centerline to match. You really want to do that at both ends.
Why graft the S-10 IFS instead of doing a full frame swap?
