The daves garage youtube has an episode where he documents the pitfalls of compiling 2bsd for a PDP-11/83. https://www.youtube.com/watch?v=IBFeM-sa2YY basically it is an art on a memory constrained system.

What I found entertaining was when he was explaining how to compile the kernel, I went Oh! that's where openbsd gets it from. it is still a very similar process.