Not long after the L31 Vortec head was introduced, there was much made of the supposed difference in flow between the Vortec heads that came with the hardened exhaust seat inserts as opposed to the heads that had induction hardened seats. This was largely due to an incorrect, though oft-repeated statement made by a car magazine that said in essence the c/n 906 head was to be avoided because of the exhaust seat insert being vastly inferior to the heads with induction hardened exhaust seats.
Despite this having been being dispelled any number of times, it still surfaces now and then. But like is often the case, there's a grain of truth to this. On c/n 906 Vortec heads having the hardened exhaust seat insert (not all c/n 906 heads used the insert), there can be a lip made by the seat where it's pressed into the head. If there's a lip it can be blended and smoothed to improve the flow using nothing more than a Dremel. This is a common procedure that's often done in performance builds- regardless of whether the seats are inserts or induction hardened or unhardened. And also on the c/n 906 heads having the insert, the seat was machined with a single angle instead of a three angle seat as seen on Vortec heads having the induction hardened exhaust seat. The wider seat was used to better transfer heat from the valve into the head/cooling system. Three angle valve seats are the norm these days, done routinely by almost every machine shop. So if the seats are reground during a valve job, the single angle seat can be made into a three angle seat without a problem.
So both the c/n 906 and c/n 062 heads have virtually the same performance potential. In fact c/n 906 heads having the hardened inserts are actually preferred by some performance engine builders, mainly for marine applications. Later production c/n 906 heads stopped using exhaust seat inserts and at some point, all Vortec head exhaust seats were induction hardened.