Better be a DAMN good reason to know that the crack at the valve seat has nothing to do with the crack at the valve seat.
The sharp edges around the valve pockets,gp bores,and nozzle bore.
Those sharp edges create a torch like hot spot where the heat comes to the edge and has no where to go.
Once the intense heat over a period of time works on these areas. The crack develops.
The crack travels into the seat area,and anywhere else. Another valve,gp bore,or nozzle bore.
The remedy is quite simple. It goes back to the days of the 318 dodge heads that had rhe same issue.
The remedy? Simply buff those sharp edges into a smooth radius. Thus not allowing heat to come to a point.
Get rid of the hot spots. Get rid of the cracks.