It's kind of funny you posted this because I have a YJ figure sitting in front of me right now, about to undergo a similar transformation. Rather than alter him to his Giant Man alter ego, I intend to put him into his Goliath outfit.
I don't own a dremel so carving off his shoulder thingies and his logo on his chest are going to have to be done manually with an exacto knife. Not sure if it will work, but what the hey. I have two extra YJ figures that are unused so they have volunteered for experimentation.