Don't think that Honda made a 550K3 in 1980. They made a 550 Highthawk, but that was a shaft drive, twin cam motor that definitely will not drop into your '76 frame.
You can put either a CB500 or a CB650 motor in a 550 frame though. The 550 exhaust will work with the 500 motor, but the CV carb'd CB650's have a sllightly different exhaust system.
TR