A CB750K8 doesn't really exist. In 1976 they stopped using the K(number) designations and went to a model year designation. I'm guessing you have a 1978 CB750K, in which case the All Balls kit should work.
You could also get the bearings from Honda, they still sell them. The part numbers you need are:
96140-63040-10 BEARING (6304U) $12.89 (2 needed, for the hub)
96140-63050-10 BEARING (6305U) $14.27 (1 needed, for the sprocket carrier)
These are Motogrid's prices. Your local dealer will probably be a little higher.
The numbers in parentheses are the bearing type, in this case 6304 and 6305. The U means they are unsealed. If you do a Google Shopping search for bearings add -2RS to the end of the number for sealed versions (6304-2RS). You should be able to find some quality bearings for not too much money. I used Nachi brand from
www.VXB.com in my 550. If you have a bearing supply place close to you they should be able to help out, too.
You may want to get a new oil seal for the sprocket carrier, too:
90753-283-000 OIL SEAL (34X55X9) $9.42
Of course, the All Balls kit already has that.
Good Luck,
~john