[84LX89GT]

If you're looking for 700-800 horsepower you're definately going to have to get a 600+ cubic inch engine with all matching components (intake manifold, headers, exhaust, ignition, etc.).
You could also get a 351 windsor based stroker engine (392 or higher cubic inches) supercharged.....there's alot of options, but all cost alot of money no matter what manufacturer you go with or what parts you put on it.
700-800 horsepower is going to be very expensive....VERY.
If you can't afford insurance for a Cobra, you definately won't be able to afford a 700-800 horsepower anything let alone mustang. Unfortunately the amount of gas it'll go through, cost of modifications, cost of maintainence, etc. will kill you in the long run. Not to mention it would be hard to drive everyday.
you can easily get a 400-500 horsepower reliable mustang, at a moderate cost (example a stock '96+ Cobra with only a supercharger). You also have to remember that braking, cooling system, suspension, frame stiffenening, and many other thinks you don't think of off hand have to be addressed in order to handle the kind of power you're thinking of.
Horsepower output of a vehicle is equal to dollar bill output of a wallet.