Does the transmission still have the wiring for the torque converter clutch hooked up? Check that somehow a path to ground is not getting to the TCC solenoid. There are too many different wiring variations to discuss, but if you just pull the connector apart...that will definitely eliminate the TCC as the problem. It should be a 4 pin plug (maybe only 3 sockets are used).
I think that starting in 1987, control over TCC engagement was primarily a function of the ECM. It relied on various inputs to decide whether conditions were right to lock the transmission up. Those inputs included: coolant temp, road speed above 35, steady output from the TPS (i.e. not accelerating or decelerating at a rapid rate), brakes not applied, and that the transmission was in at least 3rd gear.
I realize that the ECM was designed to only supply a ground path for the solenoid if & when all of the above permissives were met - but with all the changes you made, maybe something was overlooked or mis-wired.
The hot side of the solenoid is supplied with voltage through the gauge fuse and the only thing between the fuse and the positive connection of the solenoid coil is the brake switch. When the brakes are applied, the NC switch on the brake pedal arm is opened and power to the coil is cut off. And it is also probably a safe assumption that when you move the gear shift lever out of P/N, you have the brakes applied. Does it stall out if you do not apply the brakes when shifting into gear?
Of course having said all that, for the TCC to energize (in addition to possibly getting 12 VDC on the positive side) would require a ground path for the negative side.
Another possibility is that I am totally wrong - and the TCC has nothing to do with why you are stalling out.