Try this site and go to customer service for a manual. MTD Corporate :: About MTD Products
I would only use OEM belts but I am fairly sure you have at least 4 pulleys for your deck. Normally two more pulleys than the number of blades. The extra pulleys are an idler and a spring loaded tensioner that does have some adjustment. You should not have to remove the springs if you get the right belt from the start. Belt and configuration are usually a decal on the deck but that could have worn away by now.