Reno,
I may have a couple Zinsco 2 Pole devices in Storage, which I could let go for a good price.
They are from past Improvement Projects, where the Panelboards and / or Gear was removed, and replaced with more current equipment... likely due to layout revisions, etc.
The largest Panelboard Device I would have (a Device which would fit on a standard Zinsco < 225 Amp 240V Bus Kit), is 100 Amp (I think...).
Might be 70 Amp, but it seems like there was a 100 Amp 2 Pole Device in my Archives.
If the use of a non-tested Device may cause Nocturnal Horrors, the best option would be to perform a "Sub-Feed Tap", as you have mentioned.
That may be the most cost-effective choice overall.
I also agree with the option of ditching the Zinsco Panelboard, and installing something modern.
Scott
