Say a fella wants to build an SET monoblock based on the 300B..

+1 roxy 54---AK kits

In truth  you are much better to let an expert build it .
Radu Tarta has been building SET amps. And DHT preamps 
for many years ,knows many secrets that only an expert  
with tubes would know. And his prices are not that much more then you can build it yourself .there are just so many things that can go wrong . The cost of the chokes ,and transformers  are by far the most expensive part of the build just check his name out in CT .he builds for the Love of the build ,profit  is not his main goal.
he has built many amps,preamps for myself and others .well worth your time to consult with him.

Before considering building a SET amp with no expertise your 
much better off seeking an expert.
https://www.google.com/search?q=radu+tarta+amplifier&rlz=1C9BKJA_enUS788US789&oq=radu+tarta+...

The kits are one way of learning but you also get what you pay for.  Why use that tube?  What are you intending to drive?  If you are looking for a beautiful sounding tube use the 45 tube IMO.  If you are going to build an amp go Class A design.

Happy Listening.

Check out the 300B set amps on ElliotAudio.com. He’s got several designs suitable for your use.