Pistons from a 409 will not work there is absolutely nothing that is compatible between the GMC V6 and Chivy 409!
Just because there is only a slight resemblance to the shape of the valve covers doesn't mean one is based off the other
It was posted a while back which Cat Diesel Valves to use but this is for the Magnum head engines and does not apply to the 305. You can still find stock valves on ebay.