Hey Twstd EB,
Well there are two ways you could go,Tickford6 way of buying a complete axle to axle from a wrecker and swappin it with your car.Depending on what you go for will depend on price.Anywhere from diff ratio selection and condition.3.23,3.08 are common and could go for around the $400-$600 but acceration and noticalbe performance gain theres the 3.45:1 fromt the XR6 which are rare and expensive$600-a possible $1000 if you get a expensive wrecker who knows what he has got.
Then theres the OP which I went for and that was to use my housing and fill it with all the right gear.Q What car is really common,in wrecking yards and you can still buy them and the important part is it carries the 3.45:1??Give up??1 VL Commodore(acca Borg Warner),this is and will be the ony Holden part I have in my car.The guy who built my diff told me that they have a harder case hardening then the later XR series of vehicle so they last longer.I don't know why this is but I belive him.I supplied the 4 pinion LSD and axles and he made it all work for me for $450 trade.This included lapping of the gears and full strip down and rebuild of the LSD,the axles had new bearings in them already as well as the LSD.It helps being in the trade.
Now before you go and buy all these parts and drop them off to your local bloke and tell him to make it work,stop.A specialist diff bloke will have more of an idea,trust me.Iam a Mechanic and I have pulled apart a couple of diffs in my time and it still amazes me how he made the VL gears fit the EA housing but it works,as I fitted the 3.45:1 transducer gear and its accurate.
The fuel issue,well you will be driving the car in the rev range it where it makes power most of the time so you don't have to rev the shit out of it to make it move.City driving low revs in 2nd instead of big revs in 1st.Highway not labouring as it is in the rev range and overtaking is a breeze.
Air filters you Tickford6 ideas is good or for the same price you can get a pod and duct some air to it,which is a free gain.Mind you this is starting to make the "box" which you don't wnat ot do.
Hope this helps as thats what we are all here for.
QLD REP 4 EA-ED club.