I had thought the 530 chain was the stock option with the 18/48 sprockets. Is the 630 what comes with the bike? What is the difference between the 530 and 630 chain?
I know the F models had a 630 chain, so I'm assuming the Ks did as well. The first number in the chain (i.e. 5 in 530) refers to the pitch - or link to link distance. 530 is 5/8", 630 is 6/8" (3/4"), etc. The rest of the number (i.e. 30 in 530) refers to the width of the rollers inside the side plates. 30 is 3/8" width.
On my '77 F I switched to 530 (I dont really see a point in doing 520 unless you're concerned with weight reduction) and put a 17T offset sprocket from Cognito Moto on the front and a Vortex 43T on the back. If you decide to do an offset for whatever reason, make sure to pay attention to chain clearance of the motor housing near the front sprocket - I had to try a couple chains before I found one that would clear.