1.75" is perfect IMO, 2" ain't any better... just louder I guess ;)
Redback/Genie/Pacemaker headers are fine extractors. I used to own a set of Genie Terminator Headers and a 2" exhaust, single muffler, and hotdog... I was heard from a LOOOONG way away.. and I gained a little more power (not too great obviously but hey, it's a K-series.. not a Gen 3 5.7 litre) :P
Hope this is a good little guide for ya :y: And welcome, Vic boy :hammer: