I've always done torque converter stall tests in third gear forward and done the boost pressures at the same time. I lock up the service brakes first, open the throttle wide open and then drop the transmission in third gear forward and watch the boost pressure come up. If there is a problem, the boost will not build to spec from that level and the engine will drop below the specified RPM. There are many reasons why an engine would not build the proper boost when trying to bring on power from the bottom of the torque range. Design, altitude, air filters, fuel supply, governor problems, fuel ratio control valves, torque converter problems, other parasitic power draws like hydraulic pumps and so on. I was in a Cummins school years ago where the instructor was able describe troubleshooting a bad torque converter when the engine would not get full speed under load. There was a formula used to determine boost pressure when the engine was far below full stall rpm. I wish I could find those notes now.