5.3 drive by wire mistery

[hemi43]

I usually post in the Blazer/Jimmy area, but figured I would ask here because of the larger audience.
I just finished a 2003 5.3 swap into a 70 Blazer about a month ago, and it runs perfect except for an odd problem. The throttle plate will hit wide open throttle (WOT) with only about 50% pedal travel, which makes for a touchy throttle response.
All parts used for the swap came from only one donor vehicle which was in perfect running condition, so mismatched parts is not the problem.
The only modifications done to the PCM is the VATS was removed. Everything else is stock.
Any help would be appreciated.
Dan