I have tried that first method to no avail.
I even jacked up the engine (mounts have a lot of give), unbolted the headpipes, and unbolted the exhaust mounting at the transmission. I was able to [barely] force the old one out, but I did not want to risk damaging the new bushings upon install forcing things in this manner, so after hours of messing with it, I decided it was easier to just give up and remove the rear manifold.
The second time I did this (friend's car-1988), I accepted defeat right away, and started by removing the manifold.
I have read that you can get more [enough?] clearance by unbolting the engine/transmission mounts, then jacking the engine up.
I think if I do another one, I will cut the bolt, flip the new one, and use a cotter pin as you describe. The hardest part would be getting proper torque on the nut, since it would be hard to get a socket in under there. This seems to be a case where the proper torque really matters...
Best of luck.
Let me know how it goes...
1986 300E 5-Speed 240k mi.