The 2004R is NOT derivative of the TH350!!!
It was developed from the TH200 three speed automatic that was developed for lighter vehicles and originally placed behind V6 engines and the smaller V8s in the 4.7 liter range. The overdrive version is slightly heavier built, but will not measure up to the 700R4 and 4L60E as far as heft and strength.
It is NOT the overdrive that needs to be disengaged! The lockup clutch in the torque converter is what must be de-activated. With proper circuitry, (which can be found by searching the internet) you can make this work. There is also a hydraulic circuit in the valve body that can keep the converter clutch from being activated in first and second gear.
In a light weight car, the 2004R can handle pretty good horsepower, given the proper modifications and some better aftermarket parts.
