Well, the plates on a 6SN7GTA/B are larger than those of the early non-A/B types, and about the same size as the ECC99. Remember when the GTA/B were designed in pre-transistor days - if they couldn't meet the published specs we would have had a lot of consumer TVs blowing up and the tube company would have gone out of business. Now, later there were higher voltage and power tubes for color TVs such as the 6BL7, etc., but color TVs needed the higher voltage and power. No indication as far as I know that the GTA/B were in any way lacking for TV applications at the time they were in widespread use.
Of course in the old days consumer equipment was designed conservatively for long life and reliability, but still, designers probably designed to, say 70-80% of max voltage and 65-70 of max power, which is what I did for the SRX revision. I grew up in the 50s when B&W TVs were very common and there were no issues with TVs blowing up, period. Also remember that later 6SN7s probably used the bigger plates just for convenience sake - no reason to keep the older lower power design in production when the A/B had the same specs. You need to compare the plates on a pre-WWII 6SN7 to a 1960s GTA/B. The 1950s GTs probably used the same plates as the GTA/B for simplicity of manufacturing - in fact, on Tubes Asylum one poster stated he saw a carton of 1960s TungSol 6SN7s of identical construction with some tubes labelled GTB and others labeled GT.
The peak plate voltage is simply an indication that the design will take that kind of voltage w/o failing, sparking, etc. For stat amps where the highest cathode to plate voltage they will see is less than 700 volts (for a +/- 350 volt supply) this means that there shouldn't be any issues in terms of the tubes being damaged by too high a voltage. So when someone says the 6NS7 is a 450 volt tube running at 700 volts - NO! It's a tube capable of withstanding 1500 volts running up to 700 volts. And in the SRX circuit it's sitting at about 300-350 volts at rest - perfectly fine.
I know you like to build lots of stuff - I suggest you build this with 6SN7GTAs, then if it sounds lousy or blows up, come back to me and we'll look into why.