A 6 volt starter will work in a 12 volt electrical system. But, if you have a truck converted to a 12 volt system, you might want to go with a 12 volt starter motor--and they are available for the mechanical shift starter (that is the technical term for the foot-stomper, masher, foot-pedal, or whatever-you-wanna-call-it type of starter on your old Chevy).
Do you know for sure that your starter motor is indeed a 6 volt starter? If it is, the conversion to a 12 volt system may have lead over time to some breakdown internally in the starter motor due to 12 volts coursing through the contacts and brushes.
Do you have lights, heater motor, everything else?
Zero power through the starter motor could be a bad ground somewhere between the hot side connection to the starter motor and the gounding battery post, either poor connection to the frame, or a poor cable/strap connection between the frame and the gounding battery post.
When you removed the electrical switch off the top of the starter motor (I presume you disconnected the hot side battery cable at the battery), and jumped from the hot battery post to the copper contact on top of the starter motor, was this with or without the starter engaged? In other words, did you have somebody in the cab mashing the starter pedal? If yes, you were testing the starter motor under load. If no, then you were testing without any significant load on the starter motor, and the following applies.
If mobody was mashing on the starter pedal, then the starter motor was not engaged with the flywheel, and the starter motor should spin freely with the direct connection. Did you see any spark or electrical arc when you did the direct connection?
If you get a spark or electrical arc, then you are electrically connected through the starter motor to ground. However, if you get a healthy arc and the starter motor does not spin, then its likely something is very wrong--like a significant short.
If you get no spark, then there is an open circuit/wire someplace--wire, brushes, commutator.
Open circuit is better, cause you can fix it easier.
Mashing the starter pedal with you foot and getting no electrical power through the starter motor is a fairly common failing in older Chevys.
You are on the right track with your diagnosis,and if you confirm that the starter motor will not spin, it sounds like the problem is internal with the starter motor.
Quick approach, if you KNOW the battery is good, is a starter motor replacement. If you feel adventurous, pull the starter motor out, give it the direct-connection bench test with the 6 volt battery. If it refuses to spin like a top, dismantle it, clean the commutator, and replace the brushes--not a difficult job, but just make sure none of the copper leads end up touching the starter motor case when you reassemble everything.
Don't listen to some of these feller that tell you to swap out your stomper starter with a solenoid type.
Despite what anyone says, that doesn't make it any easier to start you Chevy, won't make it go faster, and it's a whole lot less fun.
