Buying an HGU56. Need to determine which size to get. Head length, based on the measuring technique in the TM, is right at 8". That's on the line between medium and large. And there's a margin of error there because I'm trying to do this myself.

How much room for adjustment is there with linings and suspension systems? Should I get the Medium or the Large?

are you buying a new HGU56? And if so, from a authorized distributor? If that's the case, the distributor will help you in sizing. Gibson and Barnes use to have a sizing guideline on their website. Also, check the Gentex site for help. They have or had maintenance manuals for the SPH series and HGU series that you can download, each with sizing guidelines. Don't buy a used military helmet unless you have the ability to change out the electronics. They don't work well in civil aircraft without modifications. 

are easy to swap and readily available online.

easily adjusted for fit, makes re-sale much better.  The "large" will fit the vast majority.....ex small, small, medium, are mostly for chicks.