Spellman had 10 times the physical talent of McClellin... OT size and wingspan with LB speed. When Zo's mind was right (which was way too rare), he was virtually unblockable. Unfortunately he was also uncoachable and had absolutely no personal discipline which showed up on and off the field and eventually undid him.
I see Shea Mac's game and best case ceiling being more like Trace Armstrong though the Alex Brown comparison also works.