Thanks!
The figure with tubing ports is a thermal vacuum switch. It reacts to heat in the T-stat housing to signal, or allow, vacuum to open or close, to effect some desired result. 1975 did not have as many of these as later years.
If you live in a smog required state, this site has manuals that guide you to the correct vacuum routing scheme. Ask bd for more info.
The welded fitting is really easy. It's a 3/4" nipple that routes hot water to the heater core in the cab. Some did this when their core leaked and they wanted to stop the flow. Smarter guys just routed the hose from the manifold to the radiator, bypassing the problem.
We look forward to seeing this move from the 'rough' column to the 'sweet'.