72 340 - What Did We Miss

It doesnt happen often but its possible the balancer ring can slip on the rubber.
Also make sure someone has not mismatched the balancer and timing cover. I think 70 up they changed which side the timing marks are on the timing cover. They can interchange as long as you use the correct balancer for the timing cover. Easy way to check all this is to bring up to tdc and see if your balancer lines up at 0 or close.
This would also be a good time to make sure your intermediate shaft was indexed correctly.