Before replacing the coil _TEST_ it using any old multi - meter, even the free/$5 ones from Harbor Freight :
#1 : set meter to 20 volts DC and see how much current is actually reaching the coil via the yellow ignition wire : should be 9 volts +/- with the key on and jump to 12 volts when you turn it to the 'start' position .
#2 : disconnect the coil's wires and set the meter to OHMS, zero it and test across the two small terminals : if your truck gives 9 volts to the coil, the resistance across the two smaller terminals should be right around 1.6 OHMS (same as any old 6 volt system) .
If it feeds 12 volt to the coil in the 'run' position you'll need a
3 OHM Coil ~ this is very important .
I know you can use any old 1960's ~ 1974 GM coil on a 9 volt system but if you really need a new coil (pretty rare FWIW), go to NAPA and buy an EPOXY FILLED ONE as they never fail .
If you get 12 volts to the coil, it needs an internally ballasted one, go to NAPA and ask for a 1972 VW Beetle 1600 coil, EPOXY filed of course, it'll out last you and the rig both .
RE: the old, nasty crappy TH350 column shifter : I took mine apart and boy howdy was it ever worn out ! I needed to drive the rig so I carefully cleaned out all the old rock hard grease, dirt and rust, lint etc. lubed it all up with the correct
long fiber grease (I used white lithium Lubriplate prolly #107) , re assembled it and disconnected the tranny shift rod and Lo ! not only does it work AS NEW, it also now makes that wonderful (might be horrible to you) original "CLACK - CLACK - CLACK" sound it did when it was new and I was a Yoot in the Dealer....
Then I shimmed up the floppy shift rod and adjusted it carefully where it attaches to the bottom of the column .
The older geezers like me all heard it when I next pulled up at the Shop and nodded approvingly .
Most mechanical things on these fine rigs can be taken apart, cleaned, lubricated and adjusted to work well again ever when they look pitiful .
Of course, nothing beats a cool Locar shifter if that's your desire .
(DAMN that thing is LOUD
)