I know it was common in the 70's for some artists to lay out covers for others--Ed Hannigan for sure, and I read about Jim Starlin doing it earlier.  (And then John Romita and Marie Severin did a lot of alterations everywhere.)  Didn't Kirby lay out most of the covers in the 60's?  No big deal, but some of the covers I've posted already look like Kirby layouts as well as the pencils.

DEFENDERS #42 by Jack Kirby & Klaus Janson

Just like he did with the Sandman, Kirby takes a Spider-Man villain--the Rhino--and makes him looks VERY powerful!  And Solarr was from a non-Kirby Captain America period and he makes him impressive.  Very fun to see Kirby's take on Valkyrie and Dr. Strange, but especially Luke Cage!

DEFENDERS #43 by Jack Kirby & Al Milgrom

Another powerful take on the Hulk, different than Kirby's 60's version.  Though a Silver Age villain, Kirby didn't draw the Cobalt Man until now--and makes him look very formidable.  Fun to see Kirby's Nighthawk.

DEFENDERS #44 by Jack Kirby & Al Milgrom

I drew all the Defenders villains for fun and I was surprised to see that he has long sleeves in the story, but here Kirby shows off powerful arms that make the cover more dynamic.  More Kirby Luke Cage and Nighthawk!

DEFENDERS #45 by Jack Kirby & ?

A powerful shot of Valkyrie as Kirby makes her his own, and more good Luke Cage and Nighthawk in the background.  I do like Kirby's 70's Hulk--I wish he had done some issues!  (Red Guardian was redrawn by John Romita.)  I wonder if Kirby knew that Hellcat was Patsy Walker--a character he drew years earlier attending Reed and Sue's wedding!

From Jack Kirby Collector #29, here's the sketch Kirby was given for Defenders #45 (I think it's by Al Milgrom, but the resolution is too lousy to be certain):

And here's Kirby's pencils.  As Eric noted, Red Guardian was redrawn, and all of the faces (particularly Valkyrie's) have had some redrawing done on them too:

Here's another from JKC #29.  The cover for Invaders #32.

First is Alan Kupperberg's layout sketch:

Next are Kirby's pencils, with an inset of the redrawn version of Thor's face that appeared on the printed issue: