I have never had a blown head gasket on a D335 or D350 engine--always a cracked head when that happens. Remove the head and check it out before you spend the money on a head gasket. Finding a replacement head will be much more difficult than sourcing a head gasket.
I bought some head gaskets from CNH for the D335/D350 engines last fall. CNH part number 273668R3. They are composite gaskets now, the original metal ones are no longer available. I have not heard any complaints about the composite ones.