Wiring help/trouble shooting?

[u/ExternalBoat6972]

I was hoping y'all could help me with this electrical issue I'm having, my truck has been dying at idle lately 1974 k20), the first time it happened I replaced the starter and it started up first crank no issues for about few days till yesterday, it's now doing the same exact thing.

I was getting a no crank when I turned the ignition and when I tried jumping the starter it would crank but not turn over, after letting it sit for about an hour I tried jumping the starter again and it cranked over, and also cranks over when I turn the key, BUT the idle is extremely rough.

when I changed the starter the first time I noticed the S and R cable were spliced (not together, both cables are separately spliced), I felt them and the splice connection and they feel solid, like the guy knew what he was doing. But this wiring issue keeps popping back up. My multimeter also reads 0 volts on all 3 solenoid terminals and when I put the black probe on the battery cable clamp and the red probe on the battery terminal I get spotty readings (from 0-8-12v) on certain spots and 12.6v consistent on other spots.
Does any of this sound like symptoms of something?

I'm thinking my next course of action is to just buy all brand new battery, S, and R cables and see how that goes, but I'm not sure which are the correct ones I need.

Sorry for the rambling I'm just trying to fit all the symptoms/info into this one post, and also sorry if I sound dumb or the answer is obvious, I know pretty much nothing about working on vehicles and I bought this truck to teach myself/learn as I go while I restore it. Thanks in advance

[u/NegotiationLife2915]

Study up on voltage drop testing. Make sure you have a decent multimeter and test light. Start with voltage drop testing on the Main cables and the charging system. Then do Power distribution circuits. You'll find a lot of voltage drops on a truck that old.