The pressure tap is on the left side of the trans above the shifter. You want to make sure you see an immediate rise in pressure when the throttle is slightly advanced. That diesel has mongo torque, you don't want to fry the clutches due to low line pressure. It's cheap insurance to check your pressures. Per my ATSG reference books I have, it says for D4-3rd gear: fwd sprag holding, fwd clutch applied and 3/4 clutch applied. For D4: fwd sprag clutch NOT holding, fwd clutch applied, 3/4 clutch applied and the 2/4 band applied.
If the problem happens only when the converter is locked I would manually lock it in D3 via a manual switch you can wire in. Good old fashion troubleshooting! Running both coolers will get the temps down to reasonable levels. Run it into the radiator 1st then the aux. cooler second.

Keith