DT466 is a good little engine.
Curious about under what conditions you tested the boost pressure?
You also don't say what model the chassis is, and how much weight you're packing, ie: how many gallons of water. And what are your RPMs in say fourth gear and what is the speed? This information is important to get a picture of whats happening.
But the simplest obvious factor in determining what is going on...when you are trying to travel in 5th or 6th gears and it can't pull itself, what color is the smoke? And how much of it is there? Does it smoke when under hard acceleration in the lower gears?
Since you imply this thing has a high reverse and its an ex city delivery truck, I suspect there are some higher speed ratios involved. Most likely 5th is direct and 6th would be overdrive. I believe 210 is the lowest or one of the lowest power ratings for the 466. It's possible this thing was set up as some are for hauling potato chips or such.
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