"Grinding" is not very specific. Can you add details? Have you driven it? Is it the same in every gear?
71B's are known for destroying the adapter plate bearings. Did you find any metal on the drain plug magnet? If you've been driving it you might drain the fluid and see how it looks. Might also be worthwhile to lift the car and get underneath with the engine running to see if you can locate the noise.
Replacing the main and countershaft bearings is not very difficult. You only have to split the cases and pull the bearings off of the ends of the shafts. The adapter plate bearings are more difficult.