Been discussed before.
The F tanks and the 77-78 K tanks are the same shape size. Variances in the fuel valve. Frame mount features are the same.
75/76 K tanks are the same size but shorter. Tank length effects seat length. Earlier 550 and 500 tank all fit on to the frame in the same way, but they have different fuel selector valve assemblies.
But, all the 550/500 tanks mount on the frame the same way and fit to any other frame.