Ron, your plot seems to verify the Individual runner characteristic, i.e. MID range performance. In fact, there seems to be an almost linear relationship between RPM and displacement/cyl. volume. The 289 is about 8.5% smaller than the 342, and the 7800 or so peak is roughly8.5% more than the 6700 peak on the 342.
I remembered my Daytona Ferrari (1970) produced 385HP at around 7500 RPM) with 6 twin-choke Webers (45mm??) from 4.4 liters, i.e.367 cc/cyl. The 342/5.6L has 700cc/cyl, damn near double.
The TWM Guru, Gary Williams, said Rousch and the high power guys use the 50mm units to produce 5-600 HP, but he didn't know at what RPM. That was all I could get. He's not an engine guy.
Comp. Cams tech help said my cam should peak at 65-6600. I'm seeing 6200, so if my 48's aren't perfectly set up, I may be looking at about all there is (I run 8 K&N air cleaners on straight stacks). He had no clue why the engine wouldn't pull higher.
Finally, found an interesting web site (Brit):
http://www.jenvey.co.uk/~Tech_QA.htm
They have an excellent overview on exactly what we've been discussing, and they make the point that Weber CARBS. are not the same as FI throttle bodies.
My current thought is to try air horns (TWM has some choices, but so does Jenvey) to see if they increase the flow. Seems a cheap way to prove or disprove the theory. The first few lines on their website got my attention, 1/3 power loss due to incorrect air horn length???? Ideal length, horn lip to valve, 17" for 7000 RPM??? 13.7" for 9000 RPM??? Wow.
Mike