Thanks for the replies, guys. I see from the engine pic in this post, Dick has the (alternator) wire bundle lying on the top of the water pump housing - I’m guessing that’s going to get no hotter than the coolant 
Is that where you mean, Dick? What are those tabs for on the side of my valve cover?
Well, I got to thinking about my old sending unit, and couldn’t remember there being a gasket under it (although there must have been). So I went out to the garage to have a look at it, and lo and behold, there was a brand new retainer and gasket I had put in the same spot, that I had bought from Vans - the manufacturer of the new gas tank. The retainer and grommet I had been trying to fit, came with the new sending unit! The van’s grommet is more like a rubber band, and neatly fits in a groove around the opening for the sending unit.
All the same, I’ll slick it up with silicone spray before I give another rat-a-tat-tat.