Given the Spitfire's longstanding CoG issues, an aft fuel tank would have been a complete non-starter. In fact, moving the engine forward 18" would have allowed more tankage, and would have improved the CoG range.
And if you then really wanted to retain the Spit's marginal longitudinal stability, I guess you could then chuck in an aft tank.
Those CoG issues were largely sorted out in mid-late 41. Firstly by a temporary expedient of bob weights added to the elevator control line, later by redesigned elevator horns.
The CoG became even less of an issue with the 60 series engines (being longer and heavier) and of course even more so the Griffon.
With a rear tank(s) (actually fitted to late model IX, XVI and XIV Spits) of ranging between 66 and 75 gallons (imperial) the Spit was unstable, but so was the Mustang with its rear tank full.
The key was to burn that off first on climbs and cruise. Then it became a normal Spit again.
My calculations, based on a Mk VIII, with the leading edge tanks, showed a combat radius of 500 miles (= Berlin) was quite possible.
Jeffery Quill (the great Spit test pilot) did an example flight, which he documents in his book. Mk IX, rear tank, 90 gallon drop tank and a bob weight on the elevator control line. Flew to the North of Scotland and back with no issues (same as a Berlin round trip).
In fact you could squeeze a little more of out the Spit VIII for long range missions by, removing the 4 x .303 guns and adding another 26gallons (13 in each wing) in additional leading edge tanks.
The USAAF also flew 2 Spits across the Atlantic with modified Spits, with Mustang like double drop tanks in each wing.
There was no technical issue with a LR Spit (as I have shown before here), not a Mustang though as that was was really a VLR plane.
The range issue is about, after you have a stable aircraft and you have dropped your external tanks, how much fuel do you have left for combat and returning?
In my calculations I allowed a 15min combat time and cruise to Berlin and back.
Obviously shorter ranges would have allowed longer combat time, though ammo would have been the key issue then.
Link to late model Mk IX/XVI models 66 (imperial) gallon rear tanks:
http://www.spitfireperformance.com/spitfire9-fuelsystem-lr.jpg