DQ,
What year heads are you running on that 318? Most of the stock heads are not that great, with the exception of the '85 up swirl-port heads, and if you are running 360 heads it will be too much. I still think you are bogging the engine out with the single plane and holley. The reason the same engine performed differently in a truck with lower gears is because the lower gears allowed the engine to run in the rpm range where the single plane intake and larger carb are most efficient. I think you have two choices, either change rear end gears, or install a good dual plane intake with a small TQ or Quadrajet. I don't think the torque converter will make as much of a difference as one of these changes. Just my opinion, but keep us posted on what works for you.