Ok so I swapped my 5.0 for a new 350 punched .40 over. I got a loose connector on the Passenger side. Didn't have any sensors left over from my tear down of the 305. I am using new GM heads so there are a bunch of bung plugs all over the new block and heads..... so I can't directly compare.... but then again it looked like the old engine had hooptie head job at some time before it broke its crankshaft.

I have a knock sensor hooked up. There are two plugs on the head one on the rear near the exhaust ports and one towards the front of the head. Is this the infamous second fan switch location?

 

The secondary fan temperature switch can be mounted in any of those locations. Wherever the wire harness reaches and is most accessible would be the best choice.