Could you post a schematic? Would make it easier to identify things like V2.
For an output tube plate a 350V cap will almost certainly be too low. Possibly acceptable for a preamp tube.
Guitar amps will often use a half wave rectifier with a center tapped power transformer. You might see something like 700VCT or 350-0-350 speced for a Marshall. In that case peak voltage after rectifier could be viewed as 0.7x of the 700 or 1.4x of the 350, either way around 490V. In practice the loaded transformer will be less than this, around 1.25 or 1.3x, so you might get 450V with the aforementioned transformer. That's with diodes, a tube rectifier will drop it a bit more. Note a tube rectifier is half wave, as are two diodes. A four diode bridge is not as common in guitar tube amps.
So you definitely need high rated caps at the early positions. Marshall will often use 500V can caps. Some builders will use two 300V caps in series with resistor dividers across them.