Technical Of sbc balancers

I am wondering about sbc balancers. Mostly between genI and genII. I know the gen1 400 were externally balanced and there fore had the undercut balancer and weighted flex plate. I know the others before 1986 were internal as well. However, I have heard of post 1986 1 piece rear main units being externally balanced at the flex plate. So I am wondering if the balancers for post 1986 were neutrally balanced. I would assume they would be since most aftermarket balancers advertise to be useful up to 1994 but just want to be sure. I am using one on a 2pc main sbc. I am guessing it should be fine.....

 

Does it vibrate at a certain RPM range....if not, its good to go

 

350 SBC with one piece rear seal have no flag on rear main so need xb flywheel,but front is zero balance,only the 400 sbc had xb front and can be seen as offsite wt as part of ring