If you can't get the pulley off, and you can't adjust the tension in enough to get the belt on. Can you loosen / remove mounting bracket bolts enough to get the belt on?? How about loosening the 4 jackshaft bolts and leave the blower stationary and move the driveshaft in enough to get the belt on. If there's not enough threds on the driveshaft bolts to allow it to move enough to install the belt , maybe remove the bolts 1 at a time and temporarily replace with longer ones to keep everything together while replacing the belt then tighten up and reinstall the original bolts one at a time ???now another thought are you attempting these repairs with the driver and passenger seats still in the van???