Hmm.

Since;
Vout = 1.25V*(1+R1/R2) where R1 is 220 Ohm,
and we want Vout ~6V, let's look at the value of R2;

R2 = R1 / ( (Vo/1.25) -1)
= 58 ohms.

So why are we using a 1Kohm pot? It doesn't provide much resolution for adjustment either.

Let's assume that we're going to tie the unconnected end of the pot to the middle leg to make it fail safe at Rmax. When that happens, we presumably want Vout=5V (to match stock voltage - mildly lean, not dangerous).
That gives R2=73 ohms. So if we use a 75ohm pot then all should be dandy - good resolution, and good fail-safe.

Here's the bummer, Ratshack only sell 10K or 1K pots. And since Fusto's already bought a 1K pot, looking for a 75ohm pot is going to be a pain. Furthermore, we might start hitting power dissippation issues with those low values.

So let's keep the 1K pot, and change R1 to make the pot failsafe and gain resolution;

For failsafe, Vout=5V, R2=1Kohm.
R1=( (Vo/1.25) - 1) * R2.
R1=( (5/1.25) -1) * R2.
R1= 3Kohm.

That then gives R2 for Vout=6;
R2= R1 / ( (Vo/1.25) -1)
R2= 3K / 3.8
R2= ~790 ohms. Perfect.

Now your next challenge is finding a 3K resistor. Ratshack don't sell them.
You could just use a 3.3Kohm resistor instead.
This would give a failsafe Vo= 5.375ohm, ie a little richer than stock. This will probably be perfect, assuming that your ideal mixture is indeed around the 6V mark. However, if for some reason you need 5.2V for an ideal mixture, be aware that using the 3.3Kohm resistor would preclude this. I think that it's worth pulling the gamble handle though. You can always change the resistance afterwards.

So to recap;
Use Mike's circuit. Tie the unconnected and middle connections of the 1Kohm pot together, and use 3.3Kohm resistor instead of a 220ohm.