1998 Z71, 5.0 miss under long pull

1998 Chevy Z71, 5.0 MPFI.  The truck starts and runs great....except when under a long (3/4 to 1 mile) uphill pull.  Nearly to the top the truck will start to  miss, and if I don't back out of the throttle it will continue to worsen, almost to the point of stalling.  Rarely sets a CEL, and when it does it's always a P0305.  I attached my fuel pressure gauge and ran it outside and attached it to the wiper arm.  55 psi the entire time.  Fuel spider was replaced with an upgraded one a year age.  New distributor, new plugs and wires.  Long term fuel trim very high on bank 1 (12 to 14).  After cresting the high it will slowly smooth out and run great.  Any ideas?

Thanks 

Hi guys,

There are two PID's which doesn't match to me (Cal. Load & Map).

Depending on sea level, 83Kpa is baro pressure (i live at 1850 meters above sea level). That means cal. Load should be about 80-90%.

*What's baro pressure at idle?
*is your engine idling at 1783 RPM
*Was throttle slightly pressed at that picture?