630K6 means 63.0 inches long with 6 ribs. A 635K6 is 63.5 inches long with 6 ribs.
This is a standard size scheme. Look on the belt you have now. Other than the part number that is listed above, you should see something similar to what I've posted.
It may say 1000K6635 or 0006K635, but the 635, the k, and the 6, should all be in there in some order.
The ONLY place I could find that would get a 630, is Napa. They have them in stock.
If you need a longer belt, you would get a 640k6 (64.0 inches, 6 ribs).
Hope this helps.