First I would:
Pull the codes.
Stupid, but check to confirm the TB plate is fully open at WOT.
Check TPS to see if the voltage is smoot increase as TB opens.
Pull and check the plugs and gaps..should be 3924's and gap be .040-.-55 range
Now other possible issues:
Was the cam degreed?
What was the fuel pressure?
Did you try advancing the timing? Balancer COULD be off.
Your torque curve is nice, HP curve is a little flat...possible to ask for the sheet with a lower smoothing factor to see a more realistic representation of the graph. May help you see more clearly were power is falling off. Did you do the compression/leakdown test? Is you FP maintaining throughout the pull? and what was the pressure at? Also I wondered if the valvetrain was set properly..to tight might cause issues.
You have some good parts, just seems like something is missing here with something else. I can't see you not being able to get at least 280's with the above parts. What are simular combo's doing on that particualr dyno...hell the machine it self could be off.