Jimmy
If you don't have a diagram check out 'the old car manual project' you can download a manual for 48-51 truck this is the link to the manual transmission page

http://chevy.oldcarmanualproject.com/shop/1948_51truck/51ctsm0714.htm

You will see what the gears look like, now you will notice that there is a sheet metal shield or deflector that lies close to the bottom, what you would need to do is use your magnet in the drain plug hole to get underneath to find any particles (hopefully you don't), I know I'm a stickler and I would probably jack up one rear wheel so as to spin the tailshaft and observe what you can see turning through the inspection port. A person could go to the lengths of baring the engine over in gear to see more of the other side of the gears. This way you could listen also for any strange grinding noises etc that could give away any problems, if a bearing is failing you should be able to hear it when baring over by hand etc. good luck

I would rather be late 10 minutes in this life than 10 minutes early for the next

The reason my lady and I have been together so long is because of the kids. Neither one of us wanted sole custody