I thought I'd post a little info on towing and twins. My brother and I both have fords with 96 cummins in them, zf-6 speeds, and SB Clutches. I have a 215 pump and my brothers is a 175. We both have the PDR setup of towing twins. He is probably around 400 hp and I'm not sure where I'm at(maybe close to 500?). We both pull quit often and so far there has been no egt issues and spool up is great on both setups. One you hit about 10 psi, spool up is instant. Mine will get to peak boost faster, but that is because of my fueling. Altitude is aroud 5500 to 7000 almost all of the time and I have pulled a load through passes of around 11,000 and never had problems.
From what I have read and done, I believe the 35/3b setup is about as good as it gets for towing and reliability.