[morzh]

Ed

The voltage on the plates of the rectifier is
1) AC voltage
2) The rectifier is centertapped meaning that the actually you only get half of each winding working at the time. This is the same as non-center-tapped with a bridge rectifier. That is your 700VAC rectify as 350VAC would in full bridge.

This said, 1W film resistors from say Vishay-Dale have WV up to 500V. I use 1W.
However 1/2W WV is typical 350V. In most cases that is quite enough.

Now you have to realize what WV is, it is pretty much the breakdown voltage where after crossing it (well, there will be a good margin) the spark occurs across the part.
Which does not mean you can apply the WV or even below.

Say, ,1W 10K resistor has WV=700V but 700V will produce 49W dissipation so leave it on for 1s and the resistor will go poof!
It still makes sense as you can apply duty cycle, say, 2% and still not exceed the power rating.

In the radios there will rarely be 350V voltage across any one resistor; if it is use 1W, or as Tom suggested, two in series.