Zenith 25MC33 Set

[Electronic M]

Have you tried adjusting the centering rings (fishing line loops protruding from the yoke are how you adjust them) to move the image up then trying to set the height and linearity?
That may help.
Subbing the vertical tubes also may help.
If you can get it high enough you could adjust it to the edge of lock and try paralleling capacitors across the the existing capacitors to try shifting the operating frequency such that it can stay in sync with full screen deflection.
Voltage checks may also be worth doing...If a socket has carbonized and created a leakage current path or some similar nonsense that changing parts won't fix then voltage checks should catch it... Maybe do this before the parallel cap decade box engineering hack.