Lovely Grinding Noises

I recently restored my 74 'Cuda and have swapped just about everything. It went from a 318 with a 904 behind it to a 360 with an 833 from Passon Performance with that came with everything for the swap. It all works pretty well, but there is a lovely grinding sound that comes from under my tunnel hump and forward. Its not a I-cant-shift-competently grind; it sounds sort of like the grinding that a snowplow makes going down the road. I dont think that it should sound like that?

My best guess is it has something to do with the clutch. I shifted into third (when the grinding seems to be worse) and pushed the clutch in and no more grinding. I am guessing that the transmission is still spinning then?

My questions:
What do you think it could be?
Is my hypothesis credible and the clutch is the problem?
Is the transmission still spinning when the car is in gear with the clutch pushed in?
Author: JedIEG