Help! Serpentine Belt Replacement

Hello All,
2000 Grand Am GT here with about 70,000 miles. After hearing my car squeal and whine for about 2 days, my serp. belt came off on my way home from work. Luckily I was only a block from my house and got it home and turned off quickly. I opened my hood to see my belt hanging out, and my tensioner pulley broken apart. I went out and got a new belt, and a new tensioner assembly. I finally got everything into place - or so I thought. I can't get the belt over the last pulley. Is there a way to give some slack by yanking on the tensioner? I see there's a square drive input to use a socket/breaker bar to twist in order to get some slack, but I don't have any tools that will fit between the tensioner and the car's body. Is there another way to do this? Thanks in advance for any help/suggestions.

Yes the tensioner has a square hole for a 3/8 ratchet or a long tensioner tool that you can buy or rent.
Really inexpesive tool and usually come with several sockets so it works for more then one automobile.

But a 3/8 ratchet will work, just not alot of room.
With a short socket you should have room.

Hope that helps!