underdrive pullies/serpentine belt issues need HELP!!!

Ok, so im new to this site but have been searching around and having found anything to help me out. I have a 94 Cobra by the way. Ok, so i put a set of underdrive pullies on and im having 1 hell of a time getting the belt on . I actually cant get the belt on unless i take off the plastic idler pulley, pull back all the way on the tensioner, then put the idler back on. Something tells me i need a longer belt. Anyone else out there run into this problem before? Im afraid it might be 2 tight and dont want to wear out the bottom end from to much stress. Anyone have a part number on the belt i need and insight on what 2 do? Thanks in advance

Anyone???