I think you might have done like me and put it around the crank wrong. I did mine that way when i rebuilt it, had a 78 or so belt with a/c and could NOt get one to fit with the tensioner pulley for the non a/c setup. I went on compnine and finally realized I had it wrong, I now can use factory sized belts that the local parts store pulls up.
http://www.compnine.com/index.php?u=...01&grouptype=B
check out the 2nd pic down on the left, that's the one you want or need to go by, but if yours works, no worries, as long as you know your belt size. Just make sure your water pump is turning the right way because it should turn reverse style for serp. setups compared to the old v belt setups that turned the same as the crank.
If you have a reverse flow pump and it's not turning the right way it can overheat, and vice versa, like if you have an ol style pump on it not reverse and you got the belt ran to make it spin in reverse etc.