Well .. hmm .. do I have any advice? Actually yes. We (my brother, a good friend and myself) rode the route in 15 days, out of Spokane, in 2005. I planned carefully, worked out all the fuel stops and built in 3 extra days 'in case' something popped up. Planned average was between 350 - 400 mi per day which was given to me as a reasonable number by a fellow who had done the trip a couple of years earlier. We lost our planned buffer days on the first leg. One bike blew a water pump near Lake Louise, not much in the way of help to be had and we had to backtrack to Calgary for a fix. With our buffer days gone the mileage average went closer to 425 per day. Surprisingly, this was difficult to maintain. Even in June, it was in the 30's and 40's and wet .. a lot. Traffic is a true bear from time to time. Road construction will cost many minutes per day, sometimes hours. We spent (more than once) 20 or 30 minutes stopped at a flagman waiting for the other direction traffic to clear, then rode at 20 MPH the 5 - 10 miles that was under construction. As far as I know, all the construction is done by now, but I'm guessing not as the ALCAN heaves badly each winter and is in rough shape all the time. I believe that a 60 mi./day pace is a stretch, if achievable at all. If you do push that hard, you will be facing the same issue we did at 420/day only magnified. Plan not to sleep (you are used to sleeping in the dark...once you get toward Liard, the dark comes later and later. You'll feel OK about riding till 1, then 2, then 3 A.M. and then it's still daylight and you cannot sleep well at all. Dangerous fatigue is a real consideration on a trip like this. Plan on clogging air filters, dinging rims, shaking parts loose you didn't know would fail (I tested my KLR on railroad tracks for several miles, then went back after more loc-tite). A headlight guard is mandatory ). The 18 wheelers on the haul road kick up a lot of gravel.
Get some 'Buzz-off' gear from whoever sells it these days. When you park, strip your jacket and slip on the insect-repelling clothing. You will save yourself some rash and burning from having to use DEET 100 for 3 or 4 days between showers. Don't ever open your tent until you're ready to dive in. Mosquitos will chase in.
Don't plan on being able to see through your windshield after two or three days, the bugs are thick.
Some places we stopped were out of one or two kinds of fuel. Some places we stopped for fuel ended up being a longer stop, just to recoup some energy.
I guess I'd recommend reading up on any other folks that have taken the trip.
http://www.southbayriders.com/forums/threads/15269/, or,
http://66.211.107.116/forum/showthread.php?s=3d66ed56b1cc2688294958be652b51b1&t=21798
Stop by for some amazing pie and ice cream here
http://www.windband.org/alaska/yukon1/yukon1.htm 
I know you said you've heard it all about the 600 mi./day pace. I would nonetheless advise against it, first from a push-the-limits-of-safety standpoint and second, form the point that with that pace you will miss the stunning beauty and cool people interactions along the way. It will limit your flexibility and put a high level of stress on your group.
OK< all that is worth 2 cents ::009:: but, having done it, I look back and can even re-think some of our priorities considering some things we missed. You may never get to make this trip again (other plans) so I recommend not missing the journey just for the destination.
They have a genuine article Polar Bear Club certificate if you swim in the Arctic Ocean (we did .. the water was 31, the air was 32, the wind was 21 MPH. Yep .. that was cold.
