The Corvair was only offered from the factory with the flat 6 engine offering various levels of horsepower . There was a aftermarket conversion kit available which allowed the use of a SBC in place of the stock 6 ! Go to
http://www.corvaircorsa.com/crown1.html for the info on that ! There's also a website devoted to Corvairs which have had the V8 SBC installed and not just in the usual rear engine configuration ! Some are rear mounted , some mid engine , and some in the front engine configuration -
http://v8vairs.com/ ! Check them out !