My workaday knowledge of Tex-Mex newspaper Spanish (I live in Texas) certainly doesn't qualify me as an expert , but if I'm not mistaken the idiom
el Muerto refers to Death personified, as in the Grim Reaper, while
la muerte is just plain old biological death.
The phrase itself is Castilian. The quote and information to which I refer in my post is taken from El Caudillo: a Political Biography of Franco (J.W.D. Trythall; McGraw-Hill, 1970, LCCCN 0-107298, p. 34).
Right. El Muerto means "the dead one."