In reply to # 1500113 by TR-PI If you place the transmission in 4th gear and have someone put there foot firmly on the brake pedal, you can undo the nut with a socket and bar.
If you place your entire weight on a breaker bar, it's pretty easy to create 150-250 ft-lb of torque.
Our transmissions are fragile enough, without being subjected to far more torque than the motor produces.
If you must immobilize the crankshaft, a flywheel lock is easy to make from a piece of angle iron,
and removing the starter gives full access to the flywheel to attach it.
With the pan dropped or the motor on a stand, a common 2x4 can be inserted between crankshaft and crankcase.
Been working on these cars for nearly 50 years now ...