340 on dyno video, whats your opinion

I think of it like a cycling machine at the gym. You can increase the resistance which is just a brake on the wheel but it still feels different than actually pedaling a real bicycle up a hill. With a dyno you aren't able to put a realistic load on the engine where it has to accelerate a mass, you load it down to where it stalls with a water brake then release the load slightly so the RPMs can rise; once the load is reduced the torque input to the dyno also goes down.

IMO the most accurate way is to load the engine at preset RPMs and "connect the dots" like was done back in the days before computer- controlled dynos. They would have a giant analog dial of torque output and for example the engine would be loaded at 2000 RPM WOT, torque would be written down. Then again at 3000, then 4000 etc etc. From that the HP would be calculated by hand. IIRC Chrysler also used gigantic weighted flywheels to determine HP, we're talking a flywheel with the same rotational inertia as trying to move a loaded Imperial down the road. No brakes to absorb torque and skew the numbers, just a huge mass to spin up.