According to DarkMark's Comics Indexing Domain at
http://darkmark6.tripod.com/indexintro.html, Adventure Comics #390 (also identified as "Giant #G-69"") consists of the following stories:
"When Supergirl Played Cupid" ["Supergirl's Super Courtship"] (from Action Comics #289) This was when Supergirl tried to find a romantic interest for her cousin and located Superwoman Luma Lynai.
"The Secret Identity of Super-Horse" (from Action Comics #301) Due to a magic spell, Comet becomes human and assumes the identity of "Broncho Bill".
"Supergirl's Cowboy Hero" ["The Day Super-Horse Became Human"] (from Action Comics #311) Comet becomes a human again thanks to Circe and has a brief romance with Supergirl (as Linda Lee).
"The Great Supergirl Mirage" (from Action Comics #256) Back in her "secret weapon" days at the Midvale Orphanage, Supergirl was accidentally photographed by Dick Wilson (later Marverne), who suspects Linda. Superman builds a Supergirl robot as Linda's alibi.
"Supergirl's Wedding Day" (from Action Comics #307) Kryptonian convict Tor-An romances Supergirl as part of a plot to discredit both her and Superman. He actually gets to the altar, but then discovers Saturn Girl impersonating as the bride -- Imra having pierced Tor-An's telepathic defenses and learned his secret.