There is a plug on the drivers side of the engine. By the #6 cylinder, about 1/2 the distance between the top and bottom. Does anyone know what it's for?

Drain plug for the block, although, most times it's probably never been used.

I just looked at mine and I'm half tempted to unscrew it Just to find out if it's oil or water.

If it's oil, I might run an oil pressure guage from there instead of the other side.

It's definately water. When I removed mine I found about a pound of silt, dirt, rust, scale, crap stuff.