You can't take 170 from JB Sentral. After JB immigration, continue straight to end. Go down escalator to bus stop. The 170 stop is near the end of the escalator. You can also take Johore express or CWL to Larkin if you have bought a ticket prior in Singapore.
I have taken CW buses to JPO before after crossing custom... and yes it's the bit where we have to take escalator down to terminal. I assume that's bus terminal we are all talking about and there is only 1 terminal there right ?
Will check again next time. thanks.