Sounds like you've got a 4L60E transmission to me. The valve body is controlled by solenoids, so the ECM is in complete control of the transmission's shift points. I don't think there's any way to get around that either. You may be able to find a conversion kit of some sort, with a vaccume modulator, and valve body. The hard parts are basicly the same as a 700R4, the torque converter is probably a lock up type, with a lock up solenoid, another thing that you'll most likely have to convert. I'll ask around at work, and see if anyone knows if it's possible.