But it isn't cheap.

 

The Mustang was all about the v8 so alot of folks here don't realize
that the I6 is prime for a turbo. If the stock motor is in good condition
a mild turbo setup could be installed and it would easily run with a 289.
The I6 is low compression (ideal for a turbo) and you wouldn't have to change the suspension. If you kept the boost mild (recommended for a stock motor), you could even keep the c4 trans (or 3-speed) and stock diff if they are in good cond.
The tricky part is plumbing the exhaust to the turbo, it's better left to a pro that knows how to install a turbo. The single exhaust would be easy for them though. Check out racing shops that install turbos. If not done correctly, your motor could easily blow up or exhaust leak may cause a fire.
It would certainly be easier and most likely cheeper (but not cheep) to have a mild turbo application installed than to buy and install all the parts needed for a v8 (engine, trans, suspension, diff, wiring, etc). Usually you will also keep the MPGs of the 6.
If you're strickly a backyard mechanic with a low budget, I would'nt mess with a turbo. Just get headers and a bigger carb (or multi-carb) for the I6 and you'll get good results. Those parts exist and are easy to install.

Basicly a turbo is an almost unlimited power source. the more you put in the more you'll get out. The condition of the motor is actually the limiting factor.
A waste gate is installed to limit boost to a predetermined pressure (PSI) so you don't harm your engine.