I have an AU Fairlane with a tickford I6,

I need to replace the center muffler and was talking to a mob that do performance mods.

They were suggesting a CAT back upgrade 2.5 inch and free flow muffer and removeing the read one, advising it will improve fuel economy and a increase in torque of between 3 & 5%.

I use this car for towing a caravan around and what I am wondering is the improvement they are claiming is real and worth doing over just fitting a replacement stock muffler.

I am not interested in top end BHP only fuel and torque and not too much noise inside

They are right you will get a little more power and better fuel, but the noise will be your problem if you are not after that. You could put another decent muffler in the tail pipe to keep the noise down though.