SUPERMAN 335 (May 1979)--"Mxyzptlk Spelled Backwards is T-R-O-U-B-L-E" by Pasko, Swan and Chiaramonte
SUPERMAN 349 (July 1980) "The Turnabout Trap" by Pasko, Swan and Chiaramonte; cover art by Ross Andru and Dick Giordano:
It's Valentine's Day and Superman is hanging out with Lois, when the subject of marriage comes up again. Meanwhile, in the 5th Dimension, Mxy is engaged to marry the lovely Miss Bgbznz, but before he can say "I do," he's banished to the 3rd Dimension.
At his Fortress, the Man of Steel hopes to give Lois Lane super-powers, so he can marry her. But when he experiments on a lizard-like alien creature, that has super-powers under a yellow sun, the Action Ace gets bit.