You can get tires better than those 40 year old designed K70's.
Just use the Avon Road Rider 26 tires on the 17 inch wheels.
You can also buy a "better" bike than a 40-year-old CB750. Using that logic we should all be driving Volvos.
I am always amazed when people ignore the question, and instead tell you how what you should or shouldn't do with your bike, based on their own preferences.
The CB750 motorcycle is extremely popular 40 years later BECAUSE it was built so well and was so far ahead of its time. And they sold so many of them for so many years.
I was not ignoring your question.
If you want Dunlop K70's and will not settle for anything else, then you will have to buy whatever it requires to make that happen.
BTW the CB750A came with 17 inch Alloy wheels. Good ones.