I had the drop tank and the side tank together in my Mk2 a number of years ago.  Even with twins, I could easily manage the Melbourne - Sydney - Newcastle run without stopping....  Capacity is around 90lts and you need 2 electronic cutoffs and a T piece to connect it up.  When the first tank runs dry, switch over the the second one (you only loose about 5km/h in the change over this way if you are paying attention).  I looked at connecting the 2 together (like a Jag or a Cooper S), however the levels are different and you would overflow the drop tank....  Filling both does weigh a little bit though... Could feel it a bit in a car with only one person.
 
PS - Don't drink too much Coke to stay awake on your long trip without stops, as you may still need a personal pitstop.....

Jim, is this the MK1 with bubble arches you're planning?
If so I would go with a centre tank in the boot (the photo below is of a Capri tank in a MK1 Escort) and the works style fuel filler.

 


 
 
 

I did that same mod to my MK1 2 litre Escort rally car in the mid 70's it worked well and also gave plenty of room in the boot.
Capri tanks may be hard to get now.

Nice mod. It look great with Peugeot 206 GTI filler cap ;)

Finally found paper clip to attach pics
 

like a few others on here, my original RS had a side tank and the drop tank.  The previous owner installed it as he used it on the highway a bit.  Mine was hooked up so that when the main drop tank was low, you flicked a switch and it would pump the fuel from the side tank into the drop tank.  I never actually used it myself as a pov 18 year old I couldn't afford that much fuel at one time.

The extension on the side tank is pretty impressive.
How much extra did that give you to the tanks capacity?
 
Hey Shaun,
I will get this image of the left hand tank out of the book and put it up.
 
I do have enough side tanks and petrol cap panel recesses to do a left hand,
but I do like the thought of a inside centre tank but I would want to adapt it to external fill access.
Jim

The internally mounted aluminium tanks dont have particularly high capacity (10-12 gallons?).
 
Have you considered the possible registration issues within your state of changing the fuel tank?  I would put the coin to something else and stay standard.

Adding 100mm to the tank gives you about 20lt extra capacity. And as the tank has about 37lt usable capacity the increase is about 50% more. What you loose in boot space is not worth worrying about.

lol I cut 25mm off of mine to fit the dry sump pipework