I'm going to look at an Olds 350 engine. It's out of the car and sitting on a stand.
There are elements of a W31 on it - #6 Heads, W-350 Aluminum Intake, W25 (carb is not W31 - 7040250)
The Harmonic Balancer "looks" like the oversized W31 version, but I'm really not sure.
When I get there, I'll be able to look closer as well as measure it - 2 1/8" width.
Can you tell from these pics if it's maybe the W31 version?

 

Definitely looks like one at first glance. I'm looking for one also hope it is for your sake.

 

funny that it has a overheat tab on the core plug, usually a sign that a used engine supplier supplied the engine. They melt at about 250F