Gunk, Gumout, Easy Off or Simple Green will work. Apply the product, allow it to set for 10-15 minutes. Prior to washing it off, hit it with a wire cup brush chucked in your drill. This will break up the big stuff before you rinse and repeat. Three or four times ought to get about anything off. If not, give another go or two. I just did the same thing on the tranny hump on my '66 K-10 so I could undercoat the bottom side and prime and paint the top side.
