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

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. 

The Elekit 300B is wonderful.  But, it's not mono block.  

The Audio Note amplifier kit is granted more expensive than the other good 300b kits mentioned above. In my opinion it’s money well spent.
1 Very sturdy and high quality aluminum chassis.
2 Very good quality standard parts. High quality power supply.
3 The option to upgrade capacitors and resistors and especially the output transformers (C-cores are available)
4Tube rectified circuit
5 Proven track record x 25 years.
6 The potential to build an upper tier high end quality 300b SET.
7 300b offers more flexibility in matching speakers due to higher power compared to the 45 and 2A3 tube options.
Charles