It will be tighter since you now have a stiffer spring in it. I usually just put it on the ground and pull the moving face up towards me, it kinda twists as it pulls up, it may be hard but it does go. If you get it to pull up then hold it and ask someone to pit the belt over it and into the space. The belt will hold it open like that while you put the pulley on and then you will have the slack you need.
Or put the belt round the rear pulley, stick the pulley on the shaft and pull the belt at the end that goes over the variator which should pull it into the rear pulley. Just be carefull not to pull the bike off the stand.
It does go, quite easy once you did it a few times, just takes practice.
If you still cant get it then as a last resort you can try use a piece of wood like a wedge to softly hammer into the rear pulley and serperate it enough to get the belt in, then just hold it there and refit the pulley.
This link shows a way to do it....
http://www.battlescooter.com/26.html