Engine Vibration 3k RPM

Even if the motor was balanced to perfection with the trw pistons....... once they got swapped for the much lighter KB’s, the rotating assy would no longer be balanced.
And it’s not just a little off either.

My old 340 .030 over TRW’s weighed 889g with pins.
The KB’s are 720g.

The reciprocating end of the rods counts one for one in the bobweight formula for a V8.

So the bobweight total with the new pistons is close to 170g lighter.

With the 170g lighter bobweights installed on the crank for balancing, you would then end up having to remove about 1/2-2/3 of that amount from the crank to put it back into normal balance.

The crank is trying to counter balance 170g of effective weight that isn’t there.

Whether or not this is the vibration the OP is feeling, I can’t say.
But one thing is for sure....... if the crank wasn’t balanced to the new pistons, the balance of the rotating assy is way way farther out of spec than any factory engine would have ever been.