I have not pulled the engine but I did remove just the transmission from my M104. The bolts are tricky to get out and there was one that I couldn't see but could feel it. The secret is having enough long extensions so that you have the breaker bar or ratchet past the rear of the transmission. I didn't use a universal joint but I did use a couple of swivel extensions that allow for some misalignment. Since I was removing the transmission, I also disconnected the transmission mount and that allowed the engine and transmission to tip back and made access to the bolts a little easier.

After you remove the flex plate bolts, push the torque converter towards the transmission as far as you can (it should move about 1/2" if I remember correctly). This will make it less likely to come with the engine.

If you are thinking about replacing the converter seal as was mentioned, be sure to do some checking to make sure that you can reinstall it without damaging the new seal. The official method of installing the torque converter is to stand the transmission on end so that the converter can be lowered into position while turning it so that the inner parts engage the transmission shaft properly. Maybe someone with actual experience can comment on the practicality of installing the torque converter when the transmission is horizontal.

Good luck.