Did I hear someone say 'yoke impedance'?
You should measure the new tube's yoke impedance with an ohmmeter first, and try to use a tube that has similar yoke impedances to the original tube's yoke. Unless are swapping yokes as well?
If you are comfortable about swapping yokes, then I'm sure that I won't have to mention 'convergence' ...
Anyway, it seems that you have been lucky enough that the replacement tube actually works.
Fuzzy picture is usually a symptom of poor convergence and/or or a yoke that isn't spaced properly (ie, the triangular rubber feet near the back of the tube). Actually, the two things are related.
Many sensible people avoid convergence adjustments as fiddling with it is very time-consuming, frustrating and not necessarily fruitful. I wish you good luck and a cool hand if you choose that path.
Sooo, having said all of that, if the tube looks like sh1t on your new chassis then I suggest that you try a different tube. Zenith tubes are indeed often crap, but Orion tubes aren't necessarily a great improvement.