When you machine the seats for OS valve and cut the seat to the same depth as the old valve, you are essentially sinking the valve and hurting flow potential. So no, you wouldn't always expect the OS valve to have the same depth of cut as the original. Also by doing so, you are assuming the depth was correct to begin with and it may not be. These old heads could have had 10 or more valve jobs done in its lifetime, so it could be far from the standard you need to compare it too.
Another problem with swapping newer technology hardware into the older engines is that many of the old engines have tips on the valves that were much longer than the modern SBC valves, and even using a +.100 longer valve may help on the spring installed height, it still may not get you were you need to be geometry-wise because of the plane the valve tip it is in is still too far away from where it was originally. There is definately a balancing act between all of these parameters to find a happy medium.
