340 Questions

I have a 73' 340 for my Demon. From what I know these are externally balanced engines. I have pulled the oil pan off to check the crank and it didn't look good! There was rust and some pitting. I need to get a new crank, but I was wondering can I make this an internally ballanced engine? If so what do I need to change. When I bought the car the motor was just put in the car for looks and thrown together from parts. Well sort of there was the block with crank, rods and pistons. he put on a set of heads and intake no pushrods. Any info would be great!
Matt