Nice.Sounds like victory. For a dedicated 78 machine, it sure sounds like an economically viable success. The reason the 78 mod sounds so awkward and inconceivable with a TD-16X model, is because a 1" pulley step, (plus change) is gigantic and can hinder the speed change mechanism useless - unless designated a single speed machine. Also, at 78 RPM (technically should be 78.26 @ 60Hz) is fast enough to induce vibrations on the stock suspension system, which was not designed for such high speed. Nitpicking aside, I don't think the content of 78 records is up to snuff with the Thorens standards anyway, so what the heck! This sure sounds interesting to me.
I think the speed change mechanism *could* still work (33 to 78) if one would carefully cut away the bottom edge of the bushing to make a chamfered edge. My concern then would be that the belt moving up from the clutch onto the bushing would push the bushing off the top of the pulley; it is just a friction fit. Another issue is one that you already mentioned. The belt guard on top of the speed change lever hits the bushing as it move down from 78 to 33, so the guard would have to be removed. Since this modification is at this point completely reversible, that modification seems too much.
There is a lot of vibration in the suspension system as the platter comes up to speed, but that goes away as the speed stabilizes. I find that the turntable seems happier if I help the platter along right after I start it up. Ideally I would rather not have to do that, but that's the engineer in me talking. In reality, the turntable will get little use, so helping it spin up is a very small price to pay for being able the play the old shellac records on my good system. I suspect that helping it spin up will also be easier on all the components involved (motor, belt, suspension).
According to an RPM app on my phone, the turntable spins at exactly 78 RPM. I can't guarantee that the power in the house (built ca. 1880) is 60 Hz, so I think we call the speed close enough. It's certainly as close as can be attained with my antique phonographs. One must remember that the speed of those phonographs change throughout the playing of the record as the motor spring loses tension.
As for needing to play the "78's" at a speed other than 78 RPM, my collection consists of jazz records from the 1940's and later. It's possible there are a few earlier dates in the mix, but not many, if any. So, better/higher speed control (for my purposes) isn't necessary.
I suspect it might have to do with warping or surface artifacts in the shellac of the record at the outer edge.