Well, after doing some searching and measuring this is what I've found out:

I need a washer that's 7/16" OD and 5/16" ID (and at this point, you're probably beginning to see the problem). But the closest SAE standard washer I found was a #8 washer that would work on the OD but I'd have to drill out the ID to work. This is beyond what I could do with a drill press (I'm not able to come up with a way to safely hold down the washer while I drill) so I'm beginning to think I need to sleeve it instead. Failing that, that remaned distributor at O'Reilly's is starting to look pretty good...