Sled moves 50ft then refuses to move

If you can pull the belt off without having to open the secondary cluch up (pushing and rotating the moveable sheave) you have the wrong belt. I had a friend that had a 440ss back when that was the same setup, I remember it took a completely different belt than the indys of the day. That was the machine that was so fussy about the belts, new belt, went like stink, wear that seemed insignificant and it would be a slug. Try a brown polaris spring in the primary, we did that to his and it woke that sled up.

The primary clutch has a white spring for low elevation riding and above 5000 feet it is an orange spring.
The clutch center to center distance on your sled is 11.5" which is why you need a different belt than most Polaris sleds of that era which had center distance of 12".

Your oem drive belt is part number 3211046. Available here http://polarisdrivebelt.com/Polaris-OEM-Drive-Belt-3211046-3211046.htm

Dayco also has a replacement belt HP3024 which can be obtained at aftermarket suppliers. Found one here http://www.mfgsupply.com/snowmobile/drivebelts/daycodrivebelts/polaris-snowmobile-drive-belts/polaris-dayco-hp-belts/hp3024.html for cheaper than Dennis Kirk.

You can measure your drive belt by holding a string around the outside circumference and then measure the length. As shown in the websites above the circumference should be 46 7/16" and width should be 1 3/8".