Petr, most of the parts are SMD and already soldered to the board. That makes it very easy to add the caps, the power transistor, the current and voltage resistors and the trimpot for adjusting the correct output voltage.. That's why this board isn't sold as "empty PCB", I guess, because it needs a smd baking oven for getting all the SMD parts on the board.
I never had any problems and I use them for years in phono pre, amps, buffers. The back of the transistor is metal and needs to be insulated. But you don't need a heat sink in this case as it gets hardly warm at 5,3 mA.
It comes with a calculator written in EXCEL. You just input voltages (in/out) and it calculates the part values for you.
For the 300B, using the existing psu with 180V DC output, you need 200V caps and R3=100R/0,5W and R9=75k/1W. I always use max current setting quite a bit higher because it sounds better. Input of the reg. is hooked up to 180V of the OTL psu , output is set to143V with the trimpot. That's it.
You can also create symmetric voltages (2 rails) which is what I did with the Inverted OTL. Here is a picture of the final version:
You use 2 regulators/transformers and wire them with an artificial ground. That would also be needed for the Pinnacles in case somebody wants to try the Maida reg. there.