While you have the belt off, check your harmonic balancer to make sure the sheave is still vulcanized to the rubber damper.
I recently had a squealing/squeaking noise on another car that I attributed to the water pump. It turned out that the sheave on the crankshaft balancer had become unstuck from the rubber that is sandwiched between the pulley hub and the sheave.
It would squeal on startup and then catch after a little while.
The harmonic damper rubber in a 216/235/261 is not vulcanized. It is simply what looks like a rubber vinyl record sandwiched between two pieces of steel discs, held in place by rivets. I had to fix one that I dropped, causing a horrible wobble. I drilled out all of the rivets with a drill bit which coincidentally made a hole which a 10 mm bolt shoulder made a snug fit inside. I bolted it back together and chucked the snout in a lathe and strategically tightened the nuts, maintaining trueness until all of the nuts were securely tightened. This process is applying even pressure on the rubber disc. If the pessure is applied unevenly, The balancer will wobble.
All of this is trivial to the OP's need for a new waterpump and the correct belt, but sometimes I get carried away.
