The thermostat you picture from NAPA is a cleveland thermostat and will work fine. It is the same style I use. I do drill two 1/8 inch holes in the mounting ring to help purge air out and give it a little flow when closed.
I have an aluminum, radiator, but not the Fluidyne. I have a custom unit from Pantera performance. He has some local shop make them. I had him make a custom single pass radiator with two 1 inch cores. It works well enough, that it will keep my 375 cid turbo motor cool idling in a parade on a 90 degree day. Most of the time, I only use 1 fan and it never gets over 220. If I run both fans, it will sit at the thermostat temp (180). I have a shroud and two Mr Gasket fans on the back. One is 14" and the other is 12". I have had it installed about 8 years with no problems. Here is a pic.
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