[Arran]

 As far as I know there is nothing inherently wrong with any 6X5GT, though some will swear by the types with the "X" style plates. If you want to evaluate the risks of using a 6X5 in the Brand Z, the first thing to do is have a look at the schematic for the 8S563, then look up the maximum specs for a 6X5 and compare them. The maximum output voltage for a 6X5 is 370 vdc, at a maximum current rating is 70 ma, with a capacitive input filter, a 5Y3/5Y4/80 is 125 ma. The one you have to look at for the most part is the power output tube since that's the big current hog on the B+ normally, in this case it's a 6AC5, maximum plate voltage 250 vdc, maximum plate current 32 ma.
 I found the Nostalgia Air diagram for this set but the resolution is very grainy so I can't read the voltages on their version of the diagram. If you want to figure out whether the 6X5 might be overstressed, add up the plate and screen grid currents of the tubes by checking the operating voltages on the schematic and then by checking a tube manual to see what the screen and plate current draw is for each tube at a given voltage, or as close to it as you can get. In this case I don't think that you will need to worry about the 6X5 being overloaded, unlike some sets in their lineup it looks like the engineers who designed this model actually consulted a tube manual before designing the power supply, other then using a cheap transformer which is a common issue with all late 1930s and early 40s brand Zs.

Here is a good site to look up tube data, they even carry European and British tube specs:

http://frank.pocnet.net/sheets66.html

Regards
Arran