Just curious about this. If I took a 6.0 iron block that is relatively new, 2929 miles, and left the stock crank in it, and was wanting to add forged rods(6.125) and pistons, how much would that throw the balance off on it? RPM's would be around 6000 max.

 

I doesn't matter how many miles are on it, a re-balance is in order. You may get rods that are heavier and pistons that are lighter and the whole rod/piston/pin/ring combo weighs the same as your stock ones, but there is much more to it than that.