[Arran]

  The voltages are an important consideration, but you didn't mention anything about current ratings for this transformer. From your description it's a light to medium duty transformer even though it is inside a potted can. The typical radio power transformer has a high voltage winding somewhere between 300-0-300 and 350-0-350, 271-0-271 is rather low considering that this is with no load across the winding.
   I would guess that since this transformer has a proper 5 volt rectifier winding then the H.V winding is probably rated at 125 ma or something since the voltage is around 271 volts with no load. You could use a 5AR4 but it's probably over rated for this application, a 5Y3/5Y4/80 will handle it quite nicely. Just remember that whilst you may almost double the output voltage by using a silicon bridge rectifier and omitting the center tap you will also reduce the current output of the supply by half verses a conventional full wave setup.
Regards
Arran