I think the amp would work better with the driver C4S R1 set to just over 100 ohms in my opinion. This will bring the bias voltage up to more like 3V, and the plate voltage up to about where it would be with a 12AT7. The heatsinks that are already on that board are likely adequate for this. If you have the shunt regulated driver upgrade, that may also need some adjustment for this to work properly.