Depends how strong your floorboards are. At least the Jag lump is aluminium, so potentially not quite such a boat-anchor.
As I just said to someone who shall remain nameless - if I was to have something on the floor then either the Jag lump or (as it's more common and cheap as chips blown up) a Rover V8..
It was painful enough running a 4 pot on Webers back in 2003 when I went injected. Hate to think what it's like now, with 3 times as much engine. 
Yeah, talk of single digit economy isn't uncommon on the Cobra forum.. although most people get low teens on a cruise

I'm hoping the 2.88 diff from the V12 coupled with a 6 speed box and injection will lead to some kind of economy
and a car with a useful first gear (useful for something other than clouds of tyre smoke, anyway

)
Still.. V12 on Webers in a California rep? That'd be a sound track to die for!
P.S. Mark - I'm willing to bet it's under 600lb for the Jag V12? I'm putting an iron block boat anchor in the Dax

(granted slightly lighter as it's got aluminium heads & intake, but it's still a big ol' iron block)