Make sure the valve springs are not the stock exploder ones, if they are swap them out for a basic set of comp springs or TFS springs. Also I'd throw a solid checking lifter in there and make sure whoever did the cam swap used the correct length pushrods which I would assume they did not.
2000 Mustang GT Steeda #0048
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.