I thought I had read that with Shimano ten speed chains, older chainrings may be too fat for the narrower chain to fit. I really thought ten speed was the cutoff.

With my most recent bike, I threw on a used chain I had sitting in a bin that looked in decent shape. When I saw that it wasn't engaging with the teeth fully, I assumed it had maybe stretched too much. I got a replacement chain without even measuring it, but then the new chain was exactly the same. I went back and measured the old chain with a chain checker and sure enough, it's just fine.

The chainrings also look to be barely worn.

Was I wrong this whole time about 9 speed chains and old chainrings? Not too thrilled to have to order new chainrings for this project, especially since the nine speed is going to be just temporary.

Most vintage chainrings work fine with 9s chain. What brand of chainrings were you using?

FWIW, I use 8-speed chains for this purpose, usually SRAM PC-8xx.

Could the front chainwheel be a heftier "Fixie" version?

Only 8 speed problem I've ever had is chains riding between the outer and inner chainwheels on Victory cranks.

Two eras away, a bridge too far.
7,8 is the new 6.

I agree that the gap between chainrings can be an issue.
keep in mind that some chainrings such as TA have the teeth offset to the inside, easier to shift to the small ring and up shifting lags.

It normally isn't a problem. What chainring?

You know, I thought of this and thought there was no way, but looking again I do think it might be a track chainring. Sugino Mighty Competition. I didn't think I had any track rings!

It's the outer on a triple right now, so it'll just have to serve as a chain guard for my test ride.

A pista chainring is designed for a 1/8” width chain. 5-6 for 3/32”

measure, of course but the expectation that a 3/32” chain will work on 1/8” ring is new math.

as you are probably using two rings up front, does the chain engage either ring?