Thanks, bertsmobile. I eventually got to the problem of stretching the spring, and the ratchet tie down worked.
For others, the initial problem is putting the spring back on the swing arm. There's not enough clearance to slip the hook of the spring into the hole on the arm. The solution is to remove the pulley at the end of the arm. My short socket wrench didn't give me enough leverage, so put some WD-40 lubricant on it, and then bought an 18" long 1/2" drive socket wrench and a 3/4" deep socket to match. The nut on the pulley came off easily with new tool.
Once the pulley was removed, the arm was able to swing completely off the back part of the deck, i.e., under the seat. With the arm in this position, the hook on the spring drops right in, and then I swung the arm back onto the deck with the spring now attached to it.
The next step is to remove the securing nut on the upright bolt the other side of the spring attaches to. Same socket wrench and deep socket made it easy. Swing the arm to the bolt, and slip the remaining hook of the spring over the bolt. Put the nut back on the bolt, but don't overtighten it. If you do, the spring will be torqued such that it isn't straight anymore. You want the spring to remain straight. Put the pulley back on the end of the swing arm, if you haven't already.
The spring is now reattached. The problem now is to stretch the spring so that you can put the belt back on the pulleys.
As bertsmobile said, a ratchet tie down works very well. Not sure how strong it has to be, but I bought a 1000 lb ratchet tie down, and it was more than sufficient, i.e., didn't struggle at all.
Ok, attach one hook of the ratchet tie down to the pulley at the end of the swing arm. Turn the hook sideways. Slide it under the pulley, but on top of the arm. Hook it around the bolt the pulley sits on. Hook the other end of the ratchet tie down to the back of the mower.
When you start to ratchet the strap, it is gonna get extremely tight. Make sure the strap underneath the mower doesn't ride up on any cabling or plastic parts that might be damaged.
Ratchet the strap until the spring is stretched enough to put the belt back on all the pulleys. Only do it just enough so that releasing ratchet isn't too difficult. Route the belt, and the release the ratchet. You're done.
HTH