So the pulleys moving when you tension the belt is because you have to put it on as tight as you can with the slack bit ending up the right place... But below is how i timed up a GT last time i did it...
If not out already remove cylinder 1s spark plug, or all of them to make turning the engine over by hand easier.
Set the bottom end to TDC:
ON the engine block is a casting mark in the shape of a point, it is at 12o'clock as you look at the end of the crank. Line up the keyway nodge on the end of the crank to this cast mark as shown in this pic
larger pic
Using a small screw driver, make sure the engine is at TDC by feeling for a piston in chamber1, if you feel no piston rotate the crank another 360degrees, and feel again.
When the bottom end is set to TDC, rotate the camshaft until it reaches this point below.
There is a small notch in the camshaft pulley, it is on the outside (visible side) and on the inside (against the plastic cowling). Line this notch up with the nodge on the timing belt cover when the viewing window is fully open. You can see the nodge directly above the belt on the photo above, also shown below
Now carefully apply the belt around the camshaft pulley, then the oil pump drive pulley, the crankshaft pulley and then the belt tensioner.
Try to leave as little slack between the camshaft pulley and oil pump pulley as possible because when you take the slack of the belt up with the tensioner you dont want this to pull the belt and turn either of the two main pulleys out of time.
The amount of slack you leave on the belt is a matter of opinion, on the longest run between the top pulley and oilpump pulley i leave enough belt to almost put a step with my fingers, hard to explain, best to discuss this with someone who has done one in person.
After you timed the crank and camshaft together remove the phase sensor, below:
line up the small notch in the wheel inside the phase sensor by rotating the shaft until it lines up with the larger line in the view glass as shown below
Now place the phase sensor carefully back, being careful not to rotate the phase sensor, best to take your time and keep trying until you get it spot on, tighten the securing bolt and voila.