Has to be 35's. You have 117V supply, a small 12V tube, three large tubes (no sand rectifier yet). 12+35+35+35=117V.
The 50V tubes worked when we had non-heated rectifiers: 12+50+50=112V (near enough).
The 60V (60FX5) tubes arrived when hi-output needles meshed with hi-gain power tubes and the preamp was dropped.
In this period, both Radio Shack and Lafayette bought products from Asian factories, contracting for their logo on box and panel. Radio Shack eventually (CB boom days) dictated what these factories would make; Lafayette faded.
12ax7 was not intended for series heater wiring so it's cold resistance is relatively high. In a series circuit that means to gets a lot more than 12VAC until the resistance of the other tube heaters comes up to operating temperature. 12+35+35+35 = 117VAC.
A bit depends on the power source for the heaters.
If they just supply enough current then on power up they will limit the current a bit and stop flaring.
I designed a hybrid amp with heater and B+ running off same 12 volt dc supply. Not thinking it through I tried raising b+ so anode voltage would increase. I raised it to 30VDC and then suddenly realised the heater was at +30VDC too ! Luckily no damage was done and the amp still worked ok.
The heaters are remarkably tough.
Ya, 35C5; 35W4 cold resistance is low, RELATIVE to the 12AX7. 12AX7 are intended for transformer heater power where they do not flare as much because the heater voltage is never more than 2x 6.3VAC, but they may still flare depending on the tube maker. In a series heater circuit, the current depends largely on the other tubes. Of course the only part of the heater you can see is the part not covered by the cathode.