Heater core replacement question

[rbade12 at aol.com]

I've got the heater box out of the car and split into it's 2 halves. I've got the old leaky core out. I got it out by impatiently cutting the elbows off to get it out of it's opening in the half the elbows fit into. Figure I'd address the insertion of a new core at another time....that's now. The opening in the heater box is too narrow for the elbows to fit through, period. I'm afraid if I force it I'll break the heater box. So, others must have run into this issue. How'd the factory do it, because they had to do it once, how'd you guys do it? Thanks, Bob