[morzh]

Ed,

A variac is a handy thing to have in any radio workshop, so you might as well get it.
Then run your radio off it before you go for the trouble of making a bucking setup.
Measure the total power but also measure the temperature of the transformer with a thermocouple, both without and with the variac over time.
Give it a rest in between so you start from the same point, this will show you the difference.

The only really vulnerable part is the transformer: tubes can take the heat and even if running a tad hotter, they will still work fine, considering you don't use the radio as they used to, you might still get years from them. And unlike the transformer they are easily (and for most radios today also inexpensively) changed.
Not so with the transformer.