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?

 

Are you aware this is a rebuilder motor, so the internal components have probably been change for generic parts. A true W31 has Flat top pistons(remove some plugs, use a bore scope if you have one), 308* camshaft, big balancer. #6 heads with large valves(all 1970 heads are #6) Block should have a Lansing VIN(M), which this does. Check for a manual trans crank, doesnt mean anything one way or the other, but hard to find

 

The owner of this engine told me that it is out of his W31 that he still owns, so I'm guessing by the looks of that balancer, it's correct. It's worth the $2500 asking price for sure... coming with the intake and OIA setup.

 

Have to wonder why he'd be selling the engine if it's out of his W-31. Not original to the car, perhaps?

 

My apologies. I see now that I could have worded this much better.
I wasn't really interested in whether the engine was a "real" W-31 engine. I simply wanted to know if the Harmonic Balancer was the correct W31 version.
Thanks for the help.