belt size for 351w on 94 GT

I seem to find a lot of conflicting information regarding the belt size for this engine so I need someone to give it to me straight.. if possible

I have a 351w with my 1994 accessories. I am using all accessories. The current belt I am using was a guess as to length. I am not sure if its too small or a hair too long. The tensioner doesnt seem to like this belt. It wants to slip when I throttle it. Anyone have any suggestions?

anyone?

That is pretty much what I did. However I think the belt is a tad short. The reason being, with the stock belt and w/302, one the belt was slipped off the idler pulley, the belt would come off. The belt I have now, once it slips off the idle pulley it is still short enough to where it hangs onto the alt, smog delete pulley, crank, a/c, ps, and water pump, and tensioner. I would then have to put a wrench on the tensioner and move it about an inch to get it to slip off the alt pulley and then it will come off. Sounds confusing...