I used an SR500 rear wheel for my tracker also, the 18" diameter really opens up the tire choices. The bearings you need are a #6206, that's not the Yamaha number but rather the number all the manufacturers use. There may be some letters after that number, that says what kind and how many seals the bearing has. For instance, a 6206FF is a #6206 bearing and has rubber contact seals on both sides, a 6206SS is also a 6206 bearing but it has non contact steel seals. I used FF's on my wheel, that way I don't have to use the big external seal you see in the SR500's rear hub. You can use the SS's but then you'll need to find a seal that will fit inside the SR500 wheel and also over the XS650 spacer. I'm sure it could be done, but I used FF's on mine, left out the big seal and everything's working perfectly so far.
I'll save you some time: the rear wheel has to shift .400 of an inch to the left (sprocket) side of the bike. Have that much machined off of the XS650 sprocket spacer. You then have to have a spacer of the same thickness made for the brake caliper side. Once that is done, the rear wheel is centered in the bike and as an added bonus the sprockets line up perfectly.
I used an XS650 caliper/mount bracket with a RD400 rear rotor, I had to add .130 between the right side inner spacer (which was the stock outer spacer for the drum brake XS650 rear wheel) and the caliper bracket so that the caliper bracket was centered over the rotor, then my custom spacer went between the caliper bracket and the swingarm. You'll need to have the 650 inner bearing spacer cut down to fit between the bearings, sorry I don't have the exact dimension handy but it's the same length as the SR500 spacer. A good machine shop can do that for you. I'm not a good machine shop, I goofed, cut mine a little too short and had to add a couple of spacer washers, I tack welded those to the ends of the spacer tube. Makes it a lot easier to install! Once the spacer is in place, if you stand the wheel up it should not slide out from between the bearings and the bearings should spin freely. There's probably .010 inch of side play between the two bearings which makes it a bit easier, i.e. if the 650 spacer is cut down to where it's, say, .008 longer than the SR500 spacer it will still work just fine.
**EDIT:** Sorry, I was sending you the wrong way with the spacer dimension. The SR500 spacer has one end smaller than the other and it is a tap fit into the sprocket side bearing (which is already a 6206, BTW). There is a 'shoulder' where that spacer is machined smaller. The dimension you need to cut the 650 spacer to is from that 'shoulder' to the brake side.
pregrid, your bike is looking GREAT!
Cumugen,
The bearings I have (New) are numbered 6204.2ZR front and 6304.2ZR rear
Leslie
about three weeks I'm goin to paint my tracker I hope it comes out 1/2 as nice as yours 
