I forgot about Fred's unique application for the 6X8 here... Sometimes I believe that he could have created a great-sounding amp from a roll of duct tape, seven fish hooks and a string of Christmas tree lights. He was the True McGyver of the Tube Amplifier World.
The 6BQ5s should work well with Fred's 6X8 driver, just increase the B+ to about 325VDC, in order to get the full "benefits" of the upgraded power tubes. (more power output is always a good thing)
I wouldn't use the ST-35's "standatrd" B+ voltages in this application (360 to 380VDC), the 325V B+, along with cahhode biasing (use a shared 150 ohm , 5W resistor, ot separate 300-330 ohm 2W cathode resistors, bypassed with 100-220 uF / 35V) should deliver around 14-WPC, with a 6.6K to 8K A-A OPT. Or use 6GK6s, ( or 10GK6, 16GK6, 29GK6, etc) for a true "Plinkertron" design.
With fixed biasing, set the current for each tube to no more than 40 mA, else the PD will be close to the upper safe dissipation limits .
Real Radios Glow in the Dark