I'm making a few assumptions - from the original post - we are talking about the XCR600 and NOT the XCR 600 SP. Meaning that you have a XLT type motor and single pipe. Traditional Indy Wedge Hood. White hood with purple graphics (I had one for 3000 miles, loved it) - XCR SP version was a triple triple, more like a reborn Storm motor with triple pipes, right from the factory, black hood.
That said, your ask about where to find specs/ideas. Forums are great, but sometimes it gets confusing. My recommendation is to start with factory info on similar sleds. go to
www.polaris.com - replacement/service parts and look at the replacement parts breakdowns - they are all on line. For example, the '97 XLT RMK would have same chassis and suspension, its a 136 (so close), same motor (maybe smaller carbs, don't remember) and see what they used for factory gearing. And the chain length required for those gears. Compare chain PN's and you will get some reassurance that you can/cannot use your original chain. Do the same for other parts, like your chain case). When I put a 144 track on my 08, I had the same tunnel and xtra 10 as your 06. So I looked at a 08 RMK - again, same tunnel, but had a 136 ext bolted on. Compare mounting points for the rear arm (function is the same, but rear is pushed back). You would also notice that you can/can't put a 2" track under the tunnel because it will rub. So you lower the mounting holes, or choose a shorter lug track )
In the end, this method takes a little time. But the factories are pretty smart on this kind of stuff. Plus you learn a ton about your sled.
You get the idea. Once you have the factory info as a baseline, then there will be literally hundreds of different opinions available on how to make it match your personality. I have found the factory recommendations to meet 99.9% of my needs.
Have fun !