[Mondial]

The voltage on the filter caps can exceed 450 V when the set is first turned on and the output tubes have not yet warmed up. The 5X4 rectifiers filaments heat almost immediately so there is full peak voltage on the filters for 30 seconds or so until the remaining tubes begin drawing current.

You can connect your voltmeter to the input filter after the rectifier and see how high the voltage rises at first turn on. I would not use caps rated at less than this maximum peak voltage. Typically, Philco used 475 V or 525 V rated caps for the first filter.