Not sure that you would want to use the 500 crank for a 350, Being heavier is not an advantage for a race engine I wouldn't think.
You also may have trouble with the balancing of the crank with the smaller 350 piston.
I have heard of racers back in the day doing the opposite in fact, and using the lighter 350 crank flywheels in a 500 engine
to make it spin up/ accelerate faster.
Not sure about how to identify a 350 as opposed to the 500, I would assume the 500 flywheels are thicker, I do have a 500 crank in a set of cases that was a race bike, but I think they have been lightened, will try and find out thru my contacts if no one else knows.
Not sure if you saw my post about my bike, on your other thread re wheel choice etc .