The starter drive isn't engaging the flywheel teeth properly. Personally I would have stayed with the 6v starter. I had a similar issue with my 53 when I converted it to 12v. My issue was the solenoid. The 6v solenoid, on 12v, would start the starter motor before the teeth were completely engaged. I put a 12v solenoid on the starter (the starter itself is still 6v) and no issue since. Your truck should have the mechanical, foot operated, solenoid though (AKA stomp starter) so that should not be an issue.

The first thing I would check is the placement of the starter switch on top of the starter. If it's too close to the device that pushes on it, it would do the same thing (start the starter spinning before it's engaged with the flywheel). If the rebuilder put the wrong switch on the starter, and the shape of the switch makes the switch contact button too close to the foot device, the result would be the same. To me it almost has to be that dynamic relationship between the foot device and the starter switch on the starter housing (assuming you still have the foot starter). If you've converted to electric starting, try putting a 12v solenoid on it.

Thanks for the advice tiny.. i still do have the foot starter mechanism. I took a look at where its at on top of the starter and its the same one i was using before the rebuilder repaired it . Well at least it seems like it is . It sits on top mounting with two screws and when i replaced it in the past all it looks like its doing is transfering the 12 volts to the inside of the starter correct ? So just wondering should i even start messing with that what i mean,  is there a right or wrong one ? Its not like yours where its a 12 or 6 volt selenoid right? 

If it was me I'd have someone operate the starter pedal while I watched the mechanism to see why/if the button is being pressed before the starter drive is fully engaged. If it's fully engaged when the button is pressed and still making the noise it may be a bad starter drive although a bad drive doesn't usually make a grinding/gnashing noise. At any rate, getting a good look while it's being operated may give you your answer.