Mission Accomplished! Thanks for the advice from all.

The alternator method, suggested by Zues, is the best way to tighten the belt, if your older 1988 m103 has a tensioning pulley that you suspect will fail if moved.

this is how i did it:

1) disconnected the plastic rad shroud by removing two clips and draped the shroud over the fan blades; it will not come out

2) removed 3 screws using a 5mm allen key on the front of the fan blade; they were placed on the fan like a 3 point star (like the mb logo?); i simply held the fan balde as i turned the screws; i also wd-40ied them before cranking them off. This was the hardest part of the job. The shroud can come out, but i had to rotate it a few time to get it to do so PITA; maybe it neednt come out.

3) route the belt, leave slack around the alternator.

4) remove bottom alternator bolt; loosen top bolt if the alternator wont budge; lift alternator tand get the belt on; push down on the alternator - it seems to act as a natural tensioning fulcrum. Install bottom bolt - BOBs YOUR UNCLE!

i had a minor problem getting the alternator to slide back into the bottom mounting posts, as the increasing tension warped the alignment of the alternator as it moved back into poistion: i tightened the top bolt which nearly re-alligned it and also held it, under tension, in position. i then put a very very small bend in the bottom post flange enought to let it slip past, then bolted it into position. if you have a helper, one can push the alternator down, while the other uses a crewdriver or tool to nudge the mounting post a "smidgee" and let the alt pass.

so far so good