It's easy to find the torque at any given RPM where you have a known HP value. The two are directly related. This won't give you peak TQ since that doesn't occur at the same engine speed as peak HP, but it will at least tend to get you in the right ballpark, since a lot of engines built for this sort of use (particularly diesels) tend to have fairly flat torque curves through most of their operating range.
The basic equation is always:
HP = TQ * RPM / 5250 whenever power is in HP, TQ is in ft-lb. Rearrange to find TQ and you have:
TQ = HP * 5250 / RPM
To do a couple examples from your numbers above:
...An LDT has 134 HP and 330 lb/ft according to most sources (although some vary)...
At 2600rpm (I'm assuming), TQ = (134hp) * 5250 / (2600 rpm) = 270.6 ft-lb at redline. That's down 60 ft-lb or 18% from peak, which is pretty decent. If you assume the 210hp version has the same torque curve with the boost cranked (which just increases TQ proportionally along most of the torque curve), then at redline you'd have:
TQ = 210hp * 5250 / 2600 = 424 ft-lb
Assuming the same ratio of redline to peak torque:
TQpk = 424 ft-lb * (330/270) = 518 ft-lb
Obviously that's just an estimate based on some assumptions, but it comes up with about the same number Yeager's quick check did. It does show a little more about how the power curve is really working. As long as you don't move the torque curve up or down on the engine speed graph, and just crank the boost, this is a pretty consistent and accurate way to estimate. If you move the torque curve, you can use this method to get a better estimate, assuming you have some idea what the new torque curve will look like.
Don't know if that was helpful or not this time, but sometimes that equation can be pretty useful.
