Once you venture outside of stock specifications, simple parts such as a drive belt become harder to find. In this case, a serpentine drive belt for a 351W based motor (a 408W stroker motor) was needed. The 351W is wider than a stock Mustang 302, increasing the belt length, the smog (thermactor) pump had been removed using an alternate belt routing, the factory A/C was retained and an 130 amp alternator had been installed with a different pulley size than the stock alternator pulley. After some trial and error, fitting different belts, a suitable length was determined.
The correct length in this case was 2075mm (approx 81-11/16″). This corresponds to Autozone part number Valucraft 817K6. This is also referenced as part number 6PK2075. With this length, the tensioner is left slightly above the middle of the allowable range. This should provide room for belt stretch over time.
Also tested was a 2073mm (81-5/8″) belt, which proved to be just slightly too small despite numerous attempts to stretch it into place. Given variation in parts, this length may fit on a different motor of the same configuration.
If the belt is too long, the tensioner will force the belt to touch the span between the alternator and the water pump. This was the case when a 2085mm (82-1/16″) (Autozone 820K6/6PK2085) and a 2093mm (82-3/8″) belt was trialed.