The tanks on XE601 were certainly custom built but I don't know if a "standard" tank was ever designed - maybe the specs mentioned refer to this? I haven't heard of any such tanks been fitted to outer pylons though, in fact I've never heard of any other Hunter being used in this role apart from 601.
Pontious, that's interesting you say that about napalm as I've heard various contradictory stories from other people, claiming that there was a napalm capability, even though it was officially denied. Or as you say, maybe it was just a case of using something similar? Either way, I think the "special" must have been either chemical or napalm, not nuclear. The notion of strapping a Read Beard under a Hunter is nonsensical and even if WE.177 wasn't too late in the story (which I think it must have been), there was no role for the Hunter, and the aircraft would have been pretty useless for nuclear delivery in any case. More to the point, there wasn't even any need for such a capability as there were more than enough suitable platforms (Scimitar, Canberra, TSR2, Vulcan, Buccaneer, Jaguar, Phantom, Tornado).