The easiest way to line up the timing belt right on a 2.3 is to find TDC for number one on the intake stroke, take the timing cover off, followed by the belt, then line the cam up with it's mark, then turn the auxilary shaft to either line up the teeth on the camshaft position sensor or distributor rotor with #1 plug on cap (can't remember if '90 was distributorless or not, I know '88 back had distributors), then put the belt on being damn sure not to turn any of the three pulleys at all, and then SLOWLY let the timing belt tensioner against the belt, watching for movement at all three pulleys, if you can get the belt tensioned down without moving them, you've got it, just put the timing cover back on and fire her up and see what she does. I've literally seen 2.3s run with the cam in 180* backwards (not well, and what little power a 2.3 has is gone, but it'll suprise you with it's smoothness), so double check everything before you bolt it all back together.
Why is this in a Mustang II tech forum anyway?