6V6 max rated plate volts is 350V.
6V6 max rated screen volts is 315V.
The screen dictates the absolute maximum B+
A typical application of 6V6 push pull is 250V to 285V on the screen and the plate, with an output transformer of 8000 to 10000 Ohms.
The 5Y3 rectifier applications show a 10uF capacitor, not a 40uF cap (rather large).
The power supply is CRC, and does not have much ripple filtering.
For push pull, a good portion of the ripple is cancelled out because it is common mode on the output transformer. However, any imbalance of the tubes and transformer now appears as hum in the speaker.
You might use a 10uF input cap (easier on the 5Y3), then perhaps a 5 Henry choke, then the 40uF cap, followed by the resistor and the other 40uF cap.
This will have good ripple filtering, but will add weight and expense (choke).
Be sure to line up the choke and power transformer, so their fields do not introduce hum in the output transformer. Use 90 degree orientation relative to the output transformer, and put some space between them and the output transformer.
With about 45V drop on a 5Y3, about a 450V to a 470V Center Tap High Voltage secondary will give about 265VDC to 279VDC, including a 4V drop in the 10uF (middle of first ripple) and a 7V drop in the choke (100 Ohm DCR, and 70mA draw). You may have to adjust this some.
The old circuit uses (Shade?) feedback, similar RH SE amps feedback, only push pull feedback. Nice idea.