Headers vs 340 manifolds

Those test did not change the carb setting IIRC. If set up for the logs, headers aren't going to make as big of a change.

FAST cars are a totally different animal. Same as people that think Stock Eliminator cars are "stock" :)

If 340 manifolds were the same as headers, stock or super stock cars wouldn't run headers. Those guys are after any marginal gain.
One 340 I have ran like a dang 3 legged crippled dog with 68-70 free flowing (yeah that's it) exh manifolds. Only change was a set of header and it ran totally different, much better.

Tube compression doesn't generally have a negative change on performance. Hang low headers suck IMO.