i know where 2 454 mark IV flywheels are:D.....had to buy new ones when i went from 454 merc 420hps to 502s

I'd balance the complete rotating assembly. I've seen rods off by as much 10 - 15 grams and sometimes even more. Had one last week.
Just my .02

even though the gen IV and Gen VI flywheels look the same, and will bolt up with no issues you will end up with a vibration doiing this..I found out the hard way...what they did on the gen V, VI was ad more wieght to the flywheel to make uo for what they took away from the crank flange to use the one peice rear main seal..Rob