Sounds to me like you would be going to great pains just to be different and to use an engine you like. IMHO what you will have in the end won't necessarily be much if any better cost or performancewise than a more traditional inline four, just different. A well tuned, four cylinder can achieve those power figures pretty easily and wont be much heavier.
You say Corvair engine makes 150HP and weighs 300 lbs. The 2.0L DOHC lump in my Neon makes 150HP and weighs about the same. Sure the cooling system will add weight, but so will widening the car to make the Corvair engine fit. Just my $.02.
R-Thanks for the reply.
Everyone likes to be different. This applies to me too, I agree. I like the music of a six or an eight, it is how I feel about exotic cars. But you forget the main point: the two engines Corvair and Neon that you are comparing will not produce 150HP the same way. I doubt that the neon can crank out 150HP at 5500rpm, the Corvair does...it has to do with the the Law of the displacement. The Corvair probably displaces twice the volume of the Neon...therefore I would have to build a larger fuel tank and indeed I would be as heavy or more than you. But does it matter. The goal is to have fun, no matter how you define fun. Take care.