Is this a gas engine..?? If so, I had the same problem with my old 310B. Does your crank pulley nut look like this..??
I had to go to another forum where I wrote asking for info... After getting it off, I posted as to what I had to do, to get it loose... I copied, and am pasting here...
**I finally got it..!! Had to heat it to cherry red, after multiple tries of just getting it good and hot, and giving it shots of PB Blaster, and Kroil.
Took a 3 foot cheater on a 1/2" breaker bar, and a 3 foot bar in the other hand to keep the engine from rotating... Sure would have been handier with a helper..., LOL...
By the way..., it is a right hand thread...
Doesn't appear that there had been thread locker on the threads. Maybe just 40 years of being on there. Not sure what to torque it to..., but rest assured there will be never sieze on the threads going back on..!!**