Richard, the ARC kits look different to whats in the pics from exvitermini ... they seem to be really just a swirl pot and external header tank setup.. rather than pulling water from 3 places along the block ..
still trying to et my head around this and look at ways it can be done with the rb25 head...
main issue i have as to a reason to use a thing like this is due to the front facing plenum im using not retaining the radiator bleeder from the TB water feed, so i cant bleed the system 100% currently
my other option is to try and include something like the justjap item like below
but that wont even out the water temps from front to rear of the block...
by the looks of the ARC kit, neither do they : - well the ones on nengun dont anyway