Got my Z all boxed in and ready to go...also snagged me a set of 10-22.5's for a mere $40! Going to start mounting the front suspension and making templates to box in the rest of the frame (from the Z up)....hope you all had a great weekend!
Excellent score on those tires and wheels. That's the same size that I have.
I'll warn you though, that tire size is obsolete, not the rim but the tire. If you do like I did and build for that size, and need to find a replacement, you'll have better luck finding 295/75r22.5 tires. They're the same diameter, but almost 2 inches wider than the 10r22.5s.
Anyway, on to the centering.
Once you find the small rims that you want to use, with the right lug pattern, and hopefully a small enough diameter that they don't overlap the holes in your 22.5s, you want to make sure the backspacing is what you want it to be. Spend a lot of time with your tape measure on that part.
Then what I did was buy three or four long carriage bolts or some pieces of all-thread. They need to be around 10 or 12 inches long, and I think I used 5/16", but almost any size would work. Each end gets big ol' fender washers and wing nuts. Space out the all-thread in the spokes of your small rims, and run them through the lug nut holes of your 22.5s. Center it by eye, and tighten it down. Mount the whole thing on your axle so you can rotate it, and just loosen, make adjustments, and tighten, until it runs true. Once it spins around and looks straight, and not like a clown car, ground to the rim (not the axle) and tack weld evenly around the rim. Spin the rim again to make sure the weld didn't pull it off center.
I welded my rims with a full bead all the way around the rim. Still, if you do this, alternate back and forth across the rim with your weld, or it will probably pull off center even after it's tacked in place. If nothing else, it helps to heat the wheel evenly, so it won't be as likely to warp. Maybe you could preheat the rims, but I don't preheat anything because I'm a 2-bit hack.
It looks like you know about welding, but this is a public forum where a lot of people read about our experiences for advice on their own projects. I think the responsible thing for me to do is to at least offer the following advice.
Welding to a wheel is not something to be taken lightly, or done haphazardly. Don't accidentally strike an arc on the rim anywhere, especially near any edges around the tire mounting surface, or the lug holes on the inner rim. Arc strikes cause cracks. They aren't just harmless pock marks that we normally ignore on non-critical items.
Also, here's a fun little video for anyone thinking about welding stuff to your rims without dismounting the tire first.
http://youtu.be/HiLeji8bLOk
Another one with a deflated tire, that blew up after welding.
http://youtu.be/uQbKCd3ezrA?t=59s
Did I forget anything?