I believe it is a temperature sensor in the DC supply line to the AC compressor. It prevents the AC compressor from running on a cold engine.
Had this back in the corner for 20 years, finally got some time to play with it. Going to rebuild shortblock as stock, small cam, good heads, 4 bbl, exhaust. See what a mild 400 will do compared to a similar 350 thats in the 70 now.g'luck on your 400 project Tom! which cast code block is it?
Don't scrap its short rods; someone on fleabay or craigslist may want em.