Welcome JD, you have certainly come to the right place for advice and information.
The ultimate Cobra is the one that best fits your requirements and budget.
My only recommendation is to make sure you are thorough in your reseatch and make the right decision for yourself as it will be a large investment in time and money. But I suspect you already know that and that is why you are here.
Just a heads up on the 540, we have worked on quite a few and in my opinion they don't have as much space and leg room as you would expect. With production seats they are very tight. Don't know if they have corrected that in later versions.
Cobra body shapes vary greatly between kit manufacturers here in Australia, especially in body dimensions.
Our PACE 427 offering uses authentic body dimensions and shape, but as Merv has correctly pointed out the PACE 427 ends up larger than some of the local kit offerings. Not because the PACE 427 has been made larger, but because some of the other body shapes are shallower or narrower than an original Cobra.
Also chassis design and layout will make a big difference to cockpit space even before you look at stretched options.
We believe we have far more interior width, height and legroom compared to most if not all other local offerings.
Note sure of your timeframe, but we will start taking orders for our new PACE 427 in the next few months.
Good luck with it, you won't regret it.