This is what Ben @ eurospec told me:
"Basically the car is spot on afr on idle and cruise, but under full
throttle (with no nos) the afr is in the 13s. Its leaner than other 350s
ive run. Ideally i would like to see it running 12.5 afr. Its basically
13.5 at worst- its not horrendous, but i suspect there is something
underlying on the car- maybe it just has a really good induction kit on
there and its genuinely getting more air.
If that is the case, something like a simple afc controller will sort him
out.
On nitrous it runs a flat 12 afr- where i would like to see an 11.5. It
wont make any more power (likely it will make marginally less, but it will
be that bit cooler and therfore safer). It has no det except at 2k when you
hold it on full load, but all 350s do that.
I would just investigate the fuel system, if its A1, then prolly just an afc
controller would do, along with a very slightly bigger fuel jet for the nos
kit."