An easy and very helpful routine, even if ya don't know how to read a schematic, is to take a look at the battery start cables, ground cables. Grounds are always a problem after about ten years or so, sometimes less.

Negative cable should go to engine block. Sometimes a bolt on the starter or bellhousing. Another to the frame from block. One more from block to firewall. These are very important.

If they are original cables or even just "old" they should be replaced with new cables of adequate size. Trust Me. Get good ones, either from a tractor or truck supply or a battery cable supplier, welding shop etc. Trust Me. The generic replacement cables sold in blister pacs are just junk and cost almost as much as good ones. The terminal will fail the first time they are tightened. They use extra thick insulation to make it look like heavy gauge wire. May be OK for a 4 cylinder car, though not for a V8 truck. Enough lecture.

Clean up each cable connection point with a wire wheel or disc or whatever, clear away paint and grime and rust at frame and block and firewall. Bright shiny clean metal. Tighten securely and coat with vaseline or grease. The solenoid or starter relay itself too, needs a solid tight clean metal to metal ground to its mounting point.

Clean tight grounds = Happy Electrons. Your starter will thank you, alternator and battery will thank you, etc. The thing about vintage cars and trucks, if the operator does not make a dedicated effort to improve the vehicle grounding, there WILL be problems, guaranteed. Even invisible corrosion between connections and ground points is enough to make starting and charging problems. No electrical engineering degree required! Good grounds are a must.

Well that settles it then. No seriously, if you were to guess, how old do you think they are? About. OEM? 30 years? 20 years?

To be clear, will the starter spin the engine? Or just a "click"?

Have you tried banging on the starter? When they start to go, can usually get a few more starts out of them that way.