The grandson of honky tonk legend Hank Williams says his family's blood runs red, white and blue ... and that's why only a true American should ever play Hank in a movie. Hank Williams III tells TMZ ... producers cookin' up a Hank biopic blew it when they cast British actor Tom Hiddleston to play…

Hank Williams Jr.'s son -- Hank Williams III -- doesn't think his father should have opened his mouth about Obama yesterday ... telling TMZ most musicians are "not worthy" of a political discussion. TMZ spoke with HW3 -- and punk rock-meets-country singer -- who tells us, "The only person out…