[Arran]

The H.V winding is too low on either of those replacements, it should be a 350-0-350, the filament current ratings are fine on the second transformer though. You have to remember that there will be a 10-20 volt drop across the rectifier tube, and then another drop across the field coil of the speaker, so that 300 volts will be something like 200 volts or less by the time it gets to the screen of the power output tube. By those measurements the H.V winding itself is not shorted, is the transformer running hot as well with no load?
Regards
Arran