I have an opportunity to pick up a 1978 CB 750 K frame with title and also an 81 CB 750 solid running engine, my question is will it fit without modification or even heavy modification?
No it is not an easy drop in. Any thing can be made to fit with enough cutting and welding but why do it? The DOHC frame is a bit stronger and has better suspension and brakes. You would have to modify the wiring to fit the dohc motor as the charging and ignition are different. Pick one generation of bike and build that bike.
I'm currently running a dohc in a frame made for a sohc. I had to raise up all the rear mounts about a half inch to fit in the dohc motor because the clutch basket on a dohc is so much bulkier.