The style on my 79 and thru 85, if I remember correctly, has a normal thread on the large bolt holding the tensioner assembly to the bracket. Takes an 11/16 wrench, and normally is not a problem to r&r.
I changed the tensioner PULLEY (not the tensioner itself) on my '91 a couple of years ago and I seem to remember that the pulley itself was attached via a reverse thread.
Also, I've checked my repair manuals and they in no way indicate the the bolt that holds the entire tensioner attached to the bracket (not the pulley itself to the tensioner) is reverse thread.
Seems to me you can remove the entire tensioner assembly from the bracket by unscrewing the bolt in the normal thread way (lefty loosie).
That bolt seems sticky/ceased - you'll have to deal with it as with any other such bolt. Saturate with penetrating oil, apply pressure and some manual imact with a good tool (but don't break the bolt), and even try intelligently applying heat to the material that the bolt is thread into and then 'crack' it loose before the heat soaks into the bolt itself.
Also, once you get it apart, you'll find a sort of guide pin on the tensioner itself and it is crucial to ensure that this pin properly in place before you reinstall and tighten the bolt.