Cast crank or steel crank

I am running my 68 cuda with a 440 and an 833 4 spd and Moser Dana 60.
I bought a lakewood bellhousing, Hayes 11" SFI steel flywheel and Centerforce Dual friction clutch.
A friend has given me a stock 440 for an engine but it has a cast crank.
I've been told that I can't use my steel flywheel with a cast crank, due to external balancing vs. internal balancing. I assume the cast crank is externally balanced and the steel flywheel is for internally balanced engines. Do I need a steel crank or can I just go with a stock flywheel?

Some light shedded on this would be greatly appreciated.

The other option is I have been offered a 68 383 engine for $500.00

What should I do?

Thanks

Gary