According to Reb Brown at Comic-Con, the studios were planning two crossover movies. Captain America (Reb Brown) and Spider-Man (Nicholas Hammond) from The Amazing Spider-Man (1977). The second was Captain America (Brown) and Lou Ferrigno/Bill Bixby from The Incredible Hulk (1978). Unfortunately, neither of these ever materialized.
Reb Brown is a big Captain America fan.
According to former Universal Studios Producer Donald Jackson, at New York Comic-Con 2005. He was in charge of working on a script for a Captain America reboot of this movie in 1981. (Concept art was featured at the panel.) The movie would have starred (Jeff Bridges) as Captain America, and would have primarily focused on Captain America and Falcon (Clarence Gilyard, Jr.) in modern day, taking on Baron Zemo, and Red Skull flashbacks would have included Peter Fonda as Red Skull. However, they were unable to reacquire the rights from Marvel. Marvel sold the rights to The Cannon Group, Inc. two years later.
Reb Brown was the second actor to play Captain America. The first was Dick Purcell, who played Captain America in the 1944 serial.
Reb Brown was the only choice to play Captain America. Being a perfect fit the comic book criteria. He was also a martial artist and happened to be under contract for Universal Studios, which made this movie.