Serpentine Belt tensioner replacement

I'm replacing the belt tensioner and having trouble figuring out how to install - my Haynes service manual has no detail on how to do it. When I install the tensioner so the alignment pin is located in the appropriate mounting hole in the block, the tensioner arm is so far back (~8 o'clock) I can't thread the serpentine belt. When the tensioner is loosened to ~7 or 6 o'clock, I can obviously thread the belt, but the alignment pin is not centered in the mounting hole & there is no way I can push the tensioner arm back far enough to get the alignment pin in the hole. See pic:

This pic is viewing the back-side (or block-side) of the tensioner spring body housing with the alignment pin (or stop pin) oriented at the top. All the posts I've seen talk as if most tensioners are self adjusting - not sure what that means. The one for the Montana is a clamshell design and would appear that the clamshell rotates around the mounting bolt, putting the arm in tension against the internal spring coil.

Question is ... after threading the belt, HOW do you get the tensioner arm pushed back enough to get the alignment or locking pin on the tensioner housing to seat in the hole on the block? There must be some step I'm missing.

Any help would be greatly appreciated!

Thanks,
Tommy