You can actually replace the early "built in bolt" type of idler with the later "through bolt" type idler; all you have to do is slightly modify (drill out) the bolt hole in the bracket.
Gates 38038
Dayco 89020
AC-Delco 38038
NAPA Belts & Hoses 38038
NAPA Temp Control 209842
The only "gotchya" you might run into is that this replacement pulley might come without the round shield plates that need to go over the central bearing in order to mount the pulley properly. You may have to find them from a junkyard '76-up car. They are sort of dished washers that contact the ends of the inner race of the bearing, but not the outer race, and space the pulley correctly to line up with the belt. I'm not sure, but a bearing supply house might be able to supply such items. If you get in a real pinch, you could probably improvise with some carefully-stacked regular washers, as long as their outer diameter is small enough not to contact the bearing's outer race.