I have a jeld wen 24 x 78 x 1 3/8 inch 6 panel hollow slab door. In there enough solid interior wood for a hole to be drilled for the knob at 40" from the bottom? It has to be that height to line up with the existing door catch, which is in a steel frame.
If steel frame door is from garage to interior, you can't use the hollow core. The door needs to have certain fire rating. You can use sheet metal wrapped or solid core.
If this is interior, I would say just hang it and drill for handle where it fits. If the hole hits the hollow, you can reinforce with canned foam and trim or glue in some blocks around the hole. Tape around the hole so you don't get glue or foam on the surface.
If your door is really 78", I would measure the opening height. Usual is more than 80". Your door may leave a large gap on the bottom.