[71 super]

Do you know for sure that the timing tab is correct for the engine? Is the motor a later model? chevy used two different timing locations and it is possible that someone has switched out the timing covers on the engine- then had to re-degree the engine hence the two white marks on the balancer. Take a look at the balancer and see if you can see any rubber protruding out from the two halves, this could be the indicator you need to replace the balancer as it may have slipped. To me, it sounds like there is a mix of parts -later engine and early timing cover. By chance does the engine have a long water pump on it?