As the exhaust was off anyways I decided to go for it so bolted them on and once the g/box was back in I could then modify the existing pipes to suit.
Having done about 100ks since I find that that is much smoother revving and get another 500-750 rpm with out much effort, also cruises at 100kph around 250-300rpm lower than previous.
Will be keen to see how much more distance per tank it gives, but very happy with the mod so far, if you get the chance to do do it without much expense, go for it.
Now for a flowmaster muffler

Paul.