Re: grand marquies exhaust
My guess is that Buffalo Bill is right, but we all remember how tight money was when we were 17, and projects such as this are how you will learn.
You should first take accurate preliminary measurements. Make notes as you do. Make sure the pipes are the same diameter and in good condition. Then it's time for you to answer your own question and consider if the project is worth the effort.
Some things to consider are do you have the proper tools to do the job? Labor is the most expensive part of a job such as this and if you, or your dad, do not have a tubing cutter and welder, you will have to pay a muffler shop to finish the job.
Do not attempt to install a muffler from a dual exhaust Mustang to your single exhaust system. The muffler will not flow and you will be disappointed. If you are going to modify your exhaust, now would be a good time to install duals. Maybe you could find a good used dual exhaust system from a "retired" law enforcement or HPP equipped CV or GM to install in your car.
Good luck with your project, and don't be afraid to learn by doing. Every hot rodder I know started out just like you!