I believe I have established the fact over the years that I care about everyone that wants to keep a 351C powered vehicle on the road. I know that Marlin is of the same frame of mind.
The original 351C cooling system was advanced for its time. Most modern cooling systems mimic it. But the 351C was the first to incorporate those features (features I'm not going to elaborate on now).
Marlin is machining his parts to far closer tolerances than US Ford or Australian Ford ever conceived possible in that era; hell possibly not even today. I have Marlin's parts in front of me. They are manufactured to bitchen tolerances and quality. I can't recommend better parts to you if you want peace of mind. Marlin's parts are bad-ass and bitchen. I know you can buy a modern 351C thermostat from a corporate auto parts chain for less money. But you can't purchase a better replacement 351C thermostat (or orifice) anywhere!
If you think I'm an idiot or a greedy SOB, or that Marlin is asking too much, go and purchase an O'Reilly or NAPA thermostat, it won't damage my ego or keep me from sleeping. I truly don't want to control you. Marlin and I have no financial arrangement either. I make nothing on his sales, I am simply thankful for his efforts. He's my brother, he makes these parts because I asked him to. The correct cooling system parts resolve all the cooling system issues with front engine (non-Pantera) vehicles. Panteras need to increase coolant flow, high flow electric fans are OK, but that's not the major issue with Panteras.
These are admittedly the post happy hour mental meanderings of an elderly inebriate.