Mike, the Mellow Corn with a touch of bourbon is very good, it makes the taste "darker" and smoother but it is still corn whiskey.
I can see that aged Scotch grain whisky might taste like some corn whiskey but I think you are being too kind to the scotch.
Because, that grain whisky is distilled out at about 94% ABV - the corn whiskey at under 160. So the latter has to be more intense. But there is a connection since the scotch grain is made (or can be made) mostly from corn or maize as they call it in England. The one you tasted certainly would have been made from corn because it was much used when that distillate would have been produced (I am allowing for its age and when you tasted it some years ago). Today, most grain whisky relies I am told on wheat, so you might not see the connection much to 'ole corn. The Scotch grain I have tasted (an excellent one is put out by Compass Box) is very similar to Canadian whisky. No surprise since the 90% of so of Canadian whisky that isn't low-proof whisky is made almost exactly like Scotch grain - from mixed grains (often wheat or corn), distilled out very high and aged in reused wood.
I am thinking about using the Mellow Corn for blending however, not to make a Scotch blend, but to assist in making an American whiskey (7 Crown-type). I understand that some American whiskey uses corn whiskey in building the blend. I wonder how they do that exactly? I propose a luxury blend, say, 50% bourbons (one old one young), 15% straight rye (not old), 10% corn whiskey, 25% GNS. (Or lower the bourbon content and increase the rye but leave the other elements unchanged).
This would be quite good I think but I don't know such a plan relates to what the distillers' recipes for American whiskey say. I know of course they limit themselves to (at most) 30% straight whiskey, which is not enough, but historically blends were made with more straight than that, all the way up to 90% in fact. They must have recipe books in those labs handed down. Their content has never been disclosed as far as I know. Oh also I would not be averse to adding a small quantity of sugar or sweet wine, to marry the blend. Just a little now.