Have you confirmed that the VCV on the injection pump is actually bleeding off vacuum? I accidentally tightened the nut a bit too snugly once, and found that it would not bleed off vacuum as the rack was opened up. Perhaps the slipping shift at full throttle is due to this condition. Have you t'd a vacuum gauge into the line to see how the vacuum going to the modulator changes with acceleration/deceleration?

I do find it odd to hear you mention that adjusting the modulator does not appear to be making a difference in shift firmness - I have always noticed a change with only one or two changes in T handle position.