I pull 8k all day up and down grades and never felt this engine lacked for power. And I came from an 8.1 with a 4.10. Acceleration on short on-ramps is not an issue. I typically get 9-9.5 depending on terrain. The 3.5 does not have that nice deep roar of the 8.1 ; )
I can understand feeling pushed around with that rig. Knowing I was towing 8k I opted for Max Payload and have the heavier suspension and 3.73.
Oh, and you can find the 3.5 de-rated in the Transit 250 and 350.
I suppose different drivers and different impressions.
Take it easy,
Stu