What other GM vehicle would use this type thermostat housing?
 

This is from a ’79 truck with a 350. What other vehicle would use the same housing but without the hole for that thermoswitch? Note that the outlet faces almost straight up because there is stuff in front of it and most Chevy ones I’ve seen are angled forward. I could just put a plug in place of the switch but it would look neater if it didn’t have the hole in the first place.

http://www.fototime.com/6EC0A5D61736728/standard.jpg
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http://www.fototime.com/D22246A8661931E/standard.jpg
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And what is the best way to prevent leaks at the thermostat which is supposed to be common on these? Gasket with dressing, no gasket just RTV, etc.?

What I do to prevent leaks is dress the gasket surface flat with a large file or a flat sanding block. The ends pull down,,,,then just use a gasket.