All factory big block engines with manual transmissions had forged cranks right to the end of big block production. That's why you can't find a factory flywheel.
Your only option is to modify a neutral flywheel or verify the imbalance on a 360 flywheel and adjust if necessary.
There used to be a company that made a thin counterweight plate that you sandwiched between the crank and the flywheel but I don't think it's in production anymore.
