[bouncytruck]

I got an a similar grinding noise in my '69. It has a TH350/NP203 combo. It turned out to be the sprag gear in the tranny had exploded and sent the bearings all over inside the case.

Since I doubt that is your problem, I'd try putting it in gear and then starting the engine. The noises carry through the drive train, so if you can get a brave friend to look under the truck while you shift, it might help determine the source (transmission vs tcase)