There's never enough time to do it right the first time, but always plenty of time to do it over again. A lesson for all of us that's timeless.
This is an old, old saying, but it is surely true because too many of us get in a hurry in our zest to finish our Mustang projects that we wind up having to do much of it over again. A successful motivational speaker by the name of Tom Hopkins tells us "touch it once" and be done with it. Do it once. Do it right. And be finished with it. Use the right parts. Opt for the best quality. Don't be afraid to spend more money to get it either. In the long run, you will spend less.
One good example is exhaust system parts. Invest in the best gaskets you can buy. You will spend more, but work less in the future. Use the best fasteners while you're at it. You must have clean surfaces void of stray gasket material and carbon deposits. Tighten header bolts in baby steps, uniformly torquing fasteners to manufacturer's specifications. Use fasteners that cannot work loose. Stage 8 Locking Header Fasteners are the best fastener out there and worth every penny because they cannot work loose.
Apply this logic to every task on your Mustang. Opt for the best components. Be not afraid to spend the money. Be patient and don't get in a hurry. You will be happier in the long run.