340 A/C Delete - Need Idler Pulley!!

Welll... depending on the year, don't be surpprised if the water pump pulley doesn't line up with the alternator. The belt routing might be crank/AC/alternator and crank/water pump/power steering.
Since you're just running it on the stand, skip the alternator and just get a belt for the crank/water pump. You can size it right so you don't need a tensioner. Just run a piece of string around the 2 pullies, tie it off and take it to the parts store. Have them size it to a matching belt.