If there is not a spring loaded tensioner on the belt you can losen the mounting bolts on the alternator and release the tension enough to get the belt on then push the alternator back into position and tighten the bolts back up.
A proper fitting serpentine belt should be a little on the snug side when installing it. The belt has to be lined up with all the ridges on each pulley before it comes close to fitting under the tensioner pulley. An extra set of hands or a really long breaker bar come in handy.
Checked the length of it against the old belt. Old belt is slightly longer, inch to inch and a half, sure this is expected as it is old and stretched.
It sounds to me like you may have the wrong belt. 1" to 1 1/2" is a pretty big difference...belts stretch over time, but not that much. What is the part number on the new belt? Is the part number on the old belt still legible?
The numbers were completely gone from the existing belt.
Went to O'Reilly Auto Parts and they were very helpful. They tried the belt I had, had the same issue. Looked thru the stock and tried on 5/8 of an inch longer, still no go. Tried another that was a full inch longer and it fit.