Well we all make mistakes so pointing out the obvious is not an insult - you may have released the tensioner and fixed it in place, then put on the new belt and forgot to release it back into place... I meant forgot to return the tensioner to it's original position, and I think Klaus was just joking with you.

Is the belt going on the correct side of each pulley (smooth side against the idler, and against the tensioner, toothed aganst the other pullies)? I found when I looped the belt over the crankshaft pulley it wanted to pop off unless I pulled upwards and maintained tension on it until it was fully installed.

So the T5 has a different config than the base? What other compenents are routed on the belt? I haven't been under the hood of a T5. As far as I know only 1 configuration has been used on the 70's. Perhaps a phone call to the dealer or a local indie shop would help you figure it out?