When I replaced my the alt on my 240, that bolt would not come out, as it hit something, thereby not allowing enough room to remove it.
I was as frustrated as you seem to be. I eventually removed the other bracket it is bolted to. That one is bolted (with four bolts, IIRC) to the engine block.
I'm thinking they did it like that to prevent the bolt, were it to fall out, from damaging the fan, or more.
I reinstalled the bolt from the front and double nutted it, so far so good.

The bottom most bolt tightens into the alt itself, no nut.

__________________
83 SD

84 CD