Just for the sake of argument,let's say a well built 427W is good for 50,000 miles with routine maintinence assuming it is street driven and not drag raced and flogged to death........and in my opinion I think 50,000 miles would be very accurate,actually believe it would should go more than that,but lets use 50,000 miles as a base...........
How many miles do you really think you will drive a Cobra per year????
Your first year,it's new to you and you love it, you'll likely put anywhere from 3,000 to a high of maybe 7,500 miles on it depending on where you live and your normal climate......after the first year, your mileage will slowly decrease to probably an average of no more than 3,000 miles a year,do the math,how many years will it take you to hit 50,000 miles????
Using my numbers your looking at somewhere around 15 years!!!!!! will you still have that Cobra in 15 years??????
I wouldn't worry about the life span, just drive it/enjoy it and maintain it and it will be fine......
of course,all this is my opinion only and that and a dollar will get you a cup of coffee in downtown Marksville on any Saturday morning........
David