How do you make a low-budget film look like a million bucks? Simple. Re-use the lavish Hello, Dolly! sets.
According to the Great Movie Musical Trivia Book by Jeff Kurti, the Harmonia Gardens set was redressed for another Twentieth Century-Fox movie, Beneath the Planet of the Apes. The restaurant set was converted to a house of worship (represented as the ruins of St. Patrick's Cathedral) for radiation-scarred mutants. At the top of the stairs, where Dolly had made her grand entrance, is an altar for an A bomb!
If you look closely at the railing on the left side you can see where it matches up.