Definately needs to have been balanced. All GM's with external balancing have to be balanced when adding after market rods and pistons. The factory rods and pistons will alway be a different weight than after market products.
After market rods are usually matches sets within a couple grams and so are after market pistons. These two added together plus rings and bearings add up to the swing/bob weight that the crank needs to be balanced too, along with the flywheel and balancer. What you have is a classic case of crank harmonics/distoration which beat the mains out. Clearance could have been a little tight on the mains, but non the less it would have still beat the mains out if the crank was not balanced for the proper bob/swing weight. The stock units was meant to run max 4500rpm and factory balanced to run that. but the minute you start running 6500/7000 rpm you are in deep DOO DOO.
As mentioned before, you will have to check the alienment on the blk. now because it may be distorted, i'm a little surprised you didn't spin the bearings with that much heat involved.
I know it's a hard and expensive learned lesson, it's something the person who assembled the engine should have warned you about. I personally would have not assembled the engine for no price for a customer without having it balanced.
But atleast when you're able to get her up and running again you'll have a better engine and a faster one.
Zip.