I agree a chunk of broom handle works fine and only has to be stuck in one side to keep the gears from moving around. Though I have never had those gears move out of place anyway.
Im not a professional, only an auto repair hobbyist, but.....
I have replaced torque converters in most of the auto trannies I have taken out over the years, and the tranny usually came out because the car would no longer move the car. Sometimes I replaced the tranny with a rebuilt, or with a junkyard unit & sometimes with one that I rebuilt myself. But I dont know whether I ever had a torque converter that was actually Bad.
On the last two cars where I took the auto tranny out (both are Escorts), the torque converter was the 'lock up' type, and both were high mileage cars (187k & 192k). Since the frictional surfaces that actually do the 'lock-up' are inaccesible inside the torque converter, I didnt care to put a maybe-almost-worn-out-torque-converter back into a car where I had spent the time to rebuild the transmission. In both of those cases it was obvious why the tranny no longer worked once I had them opened up: worn out 3-4 clutch pack (very common failure in these F4EAT trannies I was told), worn out 2-4 band, etc).
So what I am saying is that if I were going to replace a torque converter, I would not put the same transmssion back in - without either rebuilding it, buying a rebuilt, or getting a hopefully-okay junkyard unit.
In the two cases where I replaced a tranny in a car that was still (barely) driveable, I put the original torque converters back in. Both were our family's Dodge minivans, the t.c's. were not the lock up type. Both vehicles continue to run fine in the more than ten years since I put rebuilt transmissions into them, and both have over 250,000 miles on the original torque converters.
Auto trannies can be fairly easy to rebuild - IF - the failure is simply worn out frictional surfaces, with no broken gears, snapped shafts, galled bushings, etc. And since I dont have the jigs for adjusting the bearing preloading shims, I do that by guesswork or leave it as-is. I dont expect the F4EATs that I rebuilt to last another 200,000 miles, but since Im retired and dont have any long commute, they will likely outlast the rest of the car.