nope.
The 77-78 gas tank shape was alike for the CB550 F models and the 77-78K models. If you wish to use the stock seat, those are your options. There are some minor petcock differences among these (two outlet vs single outlet). Though, they all fit the tanks in the same way.
The CB500-550 K tank shape prior to 77 were alike, and shorter than the later models mentioned above. But, there are petcock fitment differences, 74 and prior.
These statements apply to US model bikes.
Cheers,