Kari Kambing 40 Hari is located at Yong Peng, Johor. You can see the place from highway PLUS. It's so popular that some people I know would wake up early in the morning and drive all the way to Yong Peng (one-hour drive) just to have breakfast there.
I finally got a chance to eat there last year, a day before Hari Raya Haji. Made a quick stop before driving to Abah's hometown.
The queue was long but the service was fast. In my opinion, it is overhyped.