The power supply voltage available needs to go from 380 to 350. The Stereomour I used a total of 1200 Ohms of series resistance in the power supply to accomplish this, you could start by replacing the four 130 Ohm/2W resistors with 600 Ohm 5W resistors. There's plenty of space in the layout for this. If you find that you don't quite have room for four 5W resistors, you can leave one 130 Ohm/2W resistor in each channel and use a 1.1K dropping resistor in place of the other 130 Ohm resistor to get your voltage drop, but it should be a 10W part.

The filament voltage is dropped from 3.15V to 2.5V through two 0.15 Ohm resistors. I would try switching these to 0.25 Ohm resistors. Be prepared to check your AC voltage after you do this and adjust as necessary. If I was assigned this task, I would buy 0.22, 0.25, and 0.27 Ohm 3W resistors, but I still wouldn't be surprised if I needed a different value.

The cathode bias resistors need to be changed to 1.6K/10W resistors.

The higher inductance tap on the plate choke means disconnecting the black wire and connecting the red wire in its place for each plate choke.

The parallel feed cap should go somewhere around 8-10uF.

If you end up doing this, please post the voltages that you end up with after the changes (including your value for the filament voltage resistors and resulting AC voltage), and PJ can look it over and see what might need to be tweaked.