Mk1 Van Diff / Gearbox combo

I’m concerned at both the amount of fuel my Mk1 van consumes, and the effect running it at 4000rpm for too long, so I need some suggestions on the best way to lower the revs at higher speed in my van.  I mainly drive it to work occasionally, and living a bit out of town means anywhere I go the first 15-20kms is 100km/h speed limited.
At the moment it has a standard four speed box, with a Borg Warner diff housing a 4.11:1 ratio.  Is it best to:

1. Look for another set of diff gears and get them swapped over
2. Look for another whole diff with a different ratio and replace the whole lot
3. Find a 5 speed Sierra gearbox and fit that.

Ideally I’d like both a lower ratio such as 3.77 or 3.54 AND a 5 speed box if that’s possible – from a calculator I found online it would be possible to get 110km/h at around 3000rpm.  Not that I intend to run right at the speed limit, but it's nice to know it could make it there! At the moment my fabrication skills aren’t top notch, so any major changes to the axle housing would either cost me a few dollars or not really happen.
I throw the floor open to those with better know-how than me! 
 

Cheers jimmyd - to confirm what diff I have it's this one:

So I could just buy a good diff from a MK2 and bolt it in, no modification to the suspension mounts, etc?  Sounds like a good plan!  I'm not too fussed about getting a blinding start - the motor's not that hotted up, basically just a standard 1600 Cortina unit, that I've got a 32/36DGV to bolt on soon.
As for the five speed - I'd be happy with cutting the tunnel and gearbox mounts (I've found a few articles about how to do that).  I feel confident enough to do something like that, just not shortening axles or changing diff gears by myself!

The diffs are a straight swap try to get a 2 litre diff as it will have a 3.54:1 ratio.

Awesome, thanks guys!  Nice to know before I go and buy the wrong one!
 
Now.... does anyone have one for sale in Tassie??