Casual readers will want page 3 for a description of the different positions and page 17 for a discussion of the results. The tldr is that the best seems to be: head against the seat in front of you, arms down by the lower legs.

The fact that any brace position won't help in a head on collision and the foolish inference that it is therefore at best pointless and at worst some kind of ghoulish conspiracy (to maintain dental records, to ensure quick deaths yada yada) is a real bugbear of mine. Same goes for the criticism of "hide under a table during a nuclear attack". With any catastrophe, there are unavoidable deaths from the initial impact and loads of avoidable deaths and injuries from falling debris, glass shards, smoke inhalation etc. These measures are about the latter.