Looked over your post again, and the direct answer to your question is to thread the adjusting nut on the threaded rod before attempting to install the belt. The belt tensioner shock tends to extend, and if you don't, the bolt falls through the hole.
You will still have to manually shove the tensioner pulley to the correct position. I used the belt to help -- pull outward by the AC compressor until it's loose enough to go over.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!