The
CF6-80 family, developed about 1980 (doh!), is roughly 24" shorter than earlier versions.
Delta had experience with the DC-8-70 re-engining and in fact accomplished conversion of a dozen of their own aircraft (under license from Cammacorp, the STC holder), and proceeded to convert several dozen other DC-8's for worldwide customers. I don't think they saw a TriStar conversion as an overwhelming challenge.