The sound of the V8 is really distinctive and probably why most people fit the V8 into the cobra. It was origonally a V8. The V12 will sound right in an e-type jaguar. I'm sure the engine and running gear on the cobras that you are looking at are from a single jaguar donor. My betting is that the cars are Dax's.
They are a great kit and I'm sure the performance would be good. If you wanted to upgrade later to a large V8 lump or a roverV8 then you could do so. The one's with V8's are more money because these are the desirable engines. It's like anything else. Customers perspective is everything.
Regards
Vince@gledhill.info