Only An AMERICAN Can Play My Granddaddy

EXCLUSIVE

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 the legendary country crooner. Hiddleston's best known for his role as Loki in "The Avengers."

Hank Sr. -- known for hits like "Your Cheatin' Heart" and "Hey Good Lookin' -- died Jan. 1, 1953 at age 29 ... after years of abusing booze and Rx drugs.