Just use a belt that allows you to (1) get it on the pulley and (2) is short enough so that you have at least 1" or so of tightening ability after it's on. You may have to "bump" the belt on (putting it on the crankshaft pulley (the innermost one) and the alternator and then as far onto the fan pulley as you can get it from the alternator side--then just touching the starter. Most times it will pop on, and then you just tighten the alternator adjustment nut, and afterward, the other bolts that allow it to pivot (and tighten).

I found that the parts store was more than willing to install the pulley and woodruff key on the rebuilt alternator. They do it all the time, and takes about 3 minutes.

Good luck!

john bonds
Carolina: '82 240D 4sp 200k