Go for the big block, my friend!
There is a company by the name of Genesis Performance Castings in Indianapolis, IN. They purchased the 427 SO molds from Ford, improved it, and currently offer crate 427 SO engines in both cast iron and Aluminum. It is possible to produce aluminum 427 engines which weigh less than 400lbs, complete engine. As for installing the engine in your 'Stang, well, that depends on the year model. '67 to '73 all came with big blocks optional, mostly the 390. The mounts are the same. It is wise to weld your shock towers and install a monte-carlo bar when you put a lot of power in the old unit-body cars. You can also install frame rails and rear end links. I have a good deal of experience with the front end components, and I can tell you that nothing could be simpler than modifying the front suspension of a Mustang to hold up a big-block. Kits are available from California Mustang (where I got mine) and many other places. Upgrading to disc brakes is VERY easy, and even with a 390 installed, your pony will stop better than a comparable Camaro with a 350 (it will still be lighter!)
The biggest obstacle when going from a small block to a big block is the transmission and mount, but even that is fairly easy.
Long story short, the 427 is EXTREMELY expensive, but not the boat anchor you might think it is. Remember "Bullit"? That was McQueen's personal Mustang, with a big rompin' 390 under the hood. Properly set up, even a big block Mustang can out-corner the competition of the day. Certainly, a Charger had no chance at all. I owned one of those, too, and I can tell you that even the worst handling Mustang can easily out corner any Charger.
And I have driven the big block Mustang at speeds well over 100mph. As I said, properly set up, these are very stable cars at high speed. My '68 Cougar just gets cooler and cooler the faster I go.
Go for it, man! Slam a 390, 429, or 427 in that Mustang! You will learn a lot of curse words doing it, but once finished, you will NEVER regret it.