Kevin, check the left pulley to be sure the long end of the pulley hub is up. Left and right pulleys both mount this way.
If OK there, I think there may be a problem with the idler/tensioner arm. The arm has a bushing inside it that is supposed to be a little longer than the hub of the arm. When the attaching bolt is tightened, the arm pivots around the bushing. If the arm can't move freely the belt tension is not constant. Most often the bushing and bolt is worn and the bushing is rusted to the arm. The assembly rotates around the bolt which in turn wears the bushing hole, bolt, and top and bottom of the bushing. When the bolt is tight, the arm won't move freely.
I have had to make special washers to make the arm pivot correctly because the deck shell is worn under it. A new tensioner arm and bushing is sometimes necessary along with careful attention to the fit of the arm to the deck shell (no binding).
Another thing that is common is a rust weakened deck shell. If the spindle mounting seats get too thin, the spindle can tilt. Improper belt alignment and bad cutting performance is the result. Same thing can happen where the tensioner arm mounts. If it tilts too much, the belt rides up the idler pulley groove.
I also noticed there are two part numbers for the big double sheave on the center pulley. JD parts is not too clear on the differences, other than the serial number of the #48 deck. I don't know if they actually interchange. Hope you got the right one.
Let us know what you find.