There are two different ways to approach this...neither of which is BETTER than the other.
I personally dont like fucking with (cutting/welding) old dirty steel, so i tend to start with new shit.
on this beam, my buddy and I decided to raise the fuck out of his centerpin, he didnt want it to drag ever. Unbolt the draglink from the swing arm on the beam, then undo the pinchbolt that holds the swing arm onto the center pin. Pull the centerpin out. You can modify the stock centerpin mount, or you can build a new one like we did, to raise the pin even more. For most people, modifying the stock mount will be easier, and give you more than adequate ground clearance.
here is a picture of a stock centerpin. The best way to do this, is carefully remove the lower bushing before you do any cutting. Once removed, cut the sleeve flush with the bottom of the mount, as indicated by the line. BUT before doing so, measure exactly how much you are going to remove and write this number down somewhere. once cut, remove any burrs from inside the sleeve, and reinstall the lower bushings, taking care to make sure the grease holes/grooves align.
THEN take the amount you cut off of the sleeve, and cut it off of the top of the centerpin (side with pinchbolt goove) it is important to cut off the exact same amount as was removed from the bottom of the sleeve. BUT again, before you cut the top off, measure exactly how far down the pinchbolt groove is from the top of the pin, so that you can re-mill it in the exact same spot.
once it is cut, and a new pinchbolt groove milled, you can re-assemble everything and be done!
this is the other way of doing it, if you like to work with new metal. Its pretty self explanatory how this works, you create a new sleeve and mount, and shorten the pin accordingly.