Some people call it "cold setting" a frame, I call it pulling on it hard to open it up.
May take a second person to pull the frame dropouts apart and another to slide the hub into place. I've had to do it on probably half of the Schwinn frames I've worked on, to some extent.
I have just always assumed they're a tight fit. And that's almost always over a stock hub. The one I REALLY had to stretch open was my wife's '47 Schwinn DX, where I fitted a drum brake wheelset.
The Fairlady took the same hubs as any other Schwinn, that is Bendix RB, Bendix 70 or 76 coasters, and Sturmey-Archer AW and TCW 3-speeds.
I've had a whole bunch of 'em. (want one?)
--Rob