Novi 2000 Belt Issues

I have a question regarding the supercharger drive belt for a novi 2000 on a 1990 mustang gt. I just finised installing a new paxton novi 2000 H.O. kit on my car that I ordered from Summit Racing. The 10 rib drive belt is what I would consider loose the defelction is like 1/2 and that is with the tensioner adjusted all the way down. if i spin the supercharger pulley by hand the belt slips around the crank pulley which I am pretty sure I should not be able to do if the tension was right, plus under full boost i am only getting 6 1/2 to 7 pounds of boost. I am really hoping I am not the only person to have had this problem with a new kit. I called Paxton and the tech guy told me I had the right belt for the 8 -10 lb kit (belt 568) and that it is the same belt they have used with the novi 2000 on the foxbody 5.0's for the past 20 years without issue. he also told me the only available belt down would be the 540 which is 2.8 inchs shorter and I am pretty sure will not fit regardless of the tensioners placement. the tech guy also told me what size the pulleys should be for that belt which I can't remember at the moment I have it written down at home but i measured them and they were right. I was hoping somebody else had this problem cause I really don't know what to do to fix it. any help you guys could give me would be greatly appreciated.

I ended up getting it to work. I thought I didn't have any adjustment room left but I ended up having more but like I said in my post I would have expected to have much more room for adjustment being that I was using the kit the way paxton sent it to me. I did not change either pulley so you would think the 8 - 10 lb kit would come with a belt that did not need to be adjusted to the point were it almost bottoms out to work. I appreciate the info on NAPA I would have never thought they carried 10rib Supercharger belts. I bet they're a lot cheaper than paxton. Thanks again for the info