Gus,
you are correct that as a 'P' plater you can't drive a 260 with a V6 engine in it. Technically you can't drive a 260 with a L28 in it either, however the chances a a copper knowing the difference is quite small. The problems would arrise if something wnet wrong and you had a prang (touch wood)). If someone died as a result of the collision, whether your fault or not, the vehicle would be inspected by coppers that would know! Or if no-one was hurt, your insurance co, via the assessor who would know the difference, may well reneg on your claim. Before you say 'but I'm an excellent driver', anyone can have a collision, regardless of skill and experience (eg: sitting at the lights and some-one runs up your bum).
I agree with Zedophile, you would be better off, in my opinion, selling your current 260 and buying another one closer to standard. then as you get off your P's (if the car has survived) do it up to your own specs, if that is your want.
Does your current car have an engineers cert? Is it registered? Is it registered as a V6? Are you able to even insure it? If so, how much is it costing you? I'm prepared to bet that whilst it will cost you quite a bit to insure a Zed, it will cost even more for one that is modified. It is my advice to any young bloke that'll listen, to hold on to the dream car for 4 years until your off your P's (even 'till after 25 if you can) and drive a boring reliable car in the meantime. P's + boring car = cheaper insurance and less constabulary attention. You earn no-claim credits for the four years and then insure the cool, chick magnet (unless it's a 2+2) at rating 2 or 3 instead of 5 or 6. If it's registered under Mn'D, then you get no credits at all. You earn 1/2 credits with extended third party.
Good luck.
MaygZ
Just saw Peter's post:- It's 125kW/t
these figures all relate to unmodified cars. On your P's you can't drive a modified car. 'Modified' hasn't been defined and would technically include a change of wheels or even radio?? I don't think people would be very concerned with mods that don't affect performance, such as interior trim, colour etc, but I'm absolutely certain eye brows would raise when it comes to an engine upgrade!