Knuck, take a look at page 2 of the below link. Check out how forum member Pike did the cooling system conversion. Easy, neat, and clean. If I had a Renix (I wouldn't), that's how I would do it.
http://comancheclub.com/topic/48597-ditching-the-renix-pressure-bottle-for-a-filler-neck-in-the-upper-hose/page-2?hl=radiator