So, pull the plug on the thermostat and find an adapter to work with my brass connectors...correct?
Thanks!

The plug on the thermostat housing should be 1/4 NPT.
The items in your pic should work just from visual. The 1/4 to 1/8 NPT bushing should be the same

Agree the water pump plug is for lube for the fuel pump cam

Yep. Just take the fittings and plug to Menard's or any hardware store and get the correct adapter.

Thanks all...plug in thermostat has been removed and is different size from Gaffrig adapter...will get the right connector and hook it up...THANKS for bearing with this novice. My goal is to "do no harm" to my set-up while I learn.
Tom

Looks like 3/8 NPT, and to verify the outside diameter should be 0.675
If true need 3/8 to 1/8 NPT bushing

You were exactly right...ACE Hardware is the place!
Coming from the thermostat housing, the gauges barely move off the peg at idle on the garden hose. If I bump the throttles up to about 1200-1500 rpms I see a slight positive gauge movement (1-2 psi)...is this normal (Hardin raw water pumps on big blocks)?

Please post up your readings when you get out on the water. I have a pressure gauge mounted in the same location and don't seem to get the readings (only around 5-6 psi around 3-4k RPM) everyone else does. But I also found out my thermostat was missing too.(bought a used engine)

Installed mine on the block drain port with a Tee fitting. Had plans to move it to the T housing to compare but never did

Will do...

I wouldn't be worried. On the hose can give inaccurate reading. When on the water just note what pressure is at current state if all is cooling well and then change impeller and housing and note what new condition pressures is at. Then you will have a baseline for when the housing and impeller is deteriorating.