Need more info. Is the recip assy all matched up from a previous build? Is this a stock engine with a 6000RPM redline? Balancing is nice to do and cheap however Chrysler engines of the muscle period were notorious for having terrible balance from the factory, they were also known for kicking the crap out of and also outlasting the competition so I guess that has to tell you something.
I know everyone will now tell me what a hack I am, but thats the truth of it all.
Sheldon