Embossed panel and two-piece springs
On a 1975-era house, the old door is almost always a single steel panel with embossed raised rectangles, hung on a two-piece torsion spring and side-mounted hardware - not the center torsion bar you'd find on anything built after the 80s. That older hardware comes out as one unit, which is faster than you'd think, and the new jambs and track go up against whatever framing is there, shimmed as needed.