RE center pins are 3/8" - stock ones are 5/16" ...and the rear axle type doesn't matter although you may have to grind the head of the center pin or enlarge the hole in the spring perch so they fit correctly.
A step-by-step for replacing the center pin? Mmmkay....
1- Get the axle out of the way. This means removing the u-bolts, etc and letting the axle out of the way...no matter how you do it do it safely so that your next of kin doesn't come yelling at me.
2- Put a c-clamp on either side of the center pin to hold the springpack in place. Not too close or you'll never be able to turn the ratchet, but not too far away either because you need to hold the springpack together in the middle.
3- Remove the center pin.
4- Replace the center pin and tighten the nut. DO NOT use the center pin to pull the springpack together though but that's not a problem because you've got c-clamps doing the job for you.
5- Remove the c-clamps.
6- Reposition the axle onto the center pin.
7- Tighten the u-bolts
8- Drive
9- Tighten the u-bolts again.