Help me with my car please?
There are SO many ways you can go, and so much (or so little you may actually need) to do, that it's impossible for anyone to give you any kind of reasonable estimate beyond a general range depending upon what you hope to accomplish, the performance and quality you hope to achieve, and whether you intend to restore the vehicle as one you intend on keeping forever or you just want to make it as loud and go as fast as possible for as long as it will hold together. You could spend a few thousand, or a few tens of thousands of dollars.
If you just want "loud", you can accomplish almost as much through the exhaust system as the engine itself. The key is ensuring the vehicle remains emissions compliant in the process or all the work you do won't make a whit of difference because you won't be able to register it to operate on the roads. The simplest, cheapest, and quickest path to "loud" (or at least loud-er) is to add a resonating exhaust tip.
If this is an heirloom your uncle left you that you want to keep for sentimental reasons, you may have a fantastic opportunity, but don't expect to do it quickly or cheaply. The cost or hassle of just storing the car in a place where you can work on it can be expensive in and of itself. But if you have the desire and persistence to "do it right", you can begin by finding and joining some of the many online Mustang enthusiast forums whose members have probably done every conceivable thing to a Mustang at one time or another spanning every combination of possibilities ranging from the cheapest and quickest to the most expensive and elaborate. Many such people have restored their 1970's Mustang to showroom condition.
You can find similar enthusiast and technical forums that discuss turbocharging, exhaust, and just about every other area of specialty, both as concepts and in practice relevant to a particular make, model, and year. By joining and participating in some of these communities, you can begin to learn what's possible, what's not, and what certain things require in terms of time, money, and effort. Once you've gotten a sense of what's possible, you can then better decide exactly what you're prepared to commit to yourself to do.
Best of luck. I hope this helps.