there are times we cannot get all we might desire from the bidding. We might miss a spade contract due to our 3c first bid choice but that does not mean we should lose sight of a simple fact. There seems to be no rational way to proceed to bid 7 of anything. Given this fact, bidding 6c now might be best since there is no way partner will know that say having the DK instead of the C or S Q is a useless card for a grand. If I also held either of the black suit queens I would hum and haw around somehow (probably 5h) to let partner know I am looking for a grand. It just seem to optimistic to me to play partner for 3 specific cards that would make a grand ideal.
Bid 6c and worry what to do if the opps unexpectedly now back in with 6h