Dayco, Gates also makes Ford idler/tensioner pulleys. I found a Gates 90mm (3.50") dia with 17mm I.D (.671") This would work on the tensioner, but the pulley may hit the tensioner base. It will be close!
Here is another brand (4 seasons) that has a 6 rib idler pulley at 3.250" dia. But, it's for a wider belt..
http://www.racepages.com/products/?N...=15&submit.y=7
http://www.racepages.com/products/?N...14&submit.y=10