154X with vent kit, geared to 19 tooth with the weight put back in the clutch. As far as the weight...dealer had taken it out prior to delivery and then put it back after two blown belts (old ones). I have also removed all of the guards around the clutch except the aluminum guard.
I broke in my first 377 belt fairly easily with about 50 miles. Haven't even got a chance to break in the second, since the snow conditions have been so good. Clutches still get pretty hot after long pulls without stopping much, but the belt has over 300 miles now and still looks perfect. Just went up on the North Cascades Pass for three days. Pulled a cargo sled in and back out with about 500 lbs. on it. Stopped a few times in the 19 mile trek each way to let her rest, but was very impressed with performance.
The 377 belt held up and was put to the test on the third day. The snow was incredibly deep and I just couldn't stand to let her rest as much as I would have liked. Belt still looks perfect!!!!!!!!!!!!
Coming from putting 5,000 miles on a '02 700 summit to this first 650 miles on this sled, I love it, even with the problems (my dealer has been great so far on the fixes).