In need of wisdom/knowledge

That's a loaded question. It's hard to say. Work from the frame up could mean lots of things. All the old mustangs are going up in value. I dont know much about the 69 Mach 1 cars, but if its in good shape it could be a fun project. So this is one of those things that you have to really make the decision yourself. How mechanically inclined are you? How much are you willing to learn? Do you want to learn? What does the car mean to YOU. Unless the car is very valuable depending on what it needs to be properly restored you could get into some serious money and quickly out pace the value of the car if you are looking to sell it when you are done. If the car has sentimental value then the overall cost doesn't matter as much. Without being able to see any pictures nobody is going to really have a solid answer of how involved the restoration of the car would be.

As for the 351 being in it. If it was my car and it was a numbers matching original car. I would freshen up that motor. Maybe do a little head, cam, intake work and call it good. That's just me. Lots of people go by the mantra of "it's your car do what you want" and I agree with that. You could put many different period correct or more modern engines in the car. All depends on you and your budget and end goals.

I would suggest to start making a list of what you know the car needs. Break down what you know it will take to do the things that it will need. Like rebuild the rear axle. That would be new axle seals, pinion seal, wheel bearings, break rebuild kits, break shoes, wheel cylinders, u-joint maybe, are you going to upgrade the gear ratio. You get the point, Look into the cost of just the parts. You can do that by going to any number of online stores and just start browsing. Map out what you can do and/or want to learn to do. At the end see what the list looks like. Try to be as honest with yourself as you can. If the list looks like something you want to do, well, now you have a starting point.

Obviously major metal damage will get expensive quick, but it can all be fixed if the desire is there.

Start making a list, take some pictures and post them here! Pictures will help get you more opinions as well.

Take some before { dirty ) and after at least a wash please . We love that stuff .

a 69 mach 1 is pretty much always worth restoring. even if the rust is so bad that bird droppings are holding it together.

as to restoring the car, and perhaps upgrading systems along the way. that will depend on your personal thoughts regarding originality, and what you plan on doing with the car in the end. for instance, if you are planning on flipping this car, then freshen things up that are easily done, including the engine, keep it as stock as possible to widen your market, and then sell the car.

if you plan on keeping the car though, then build it like you want it built. i would however suggest that you keep an eye on ultimate resale value, and dont do anything to harm the ultimate value of the car. personally i like the 351w as it can make some real power with some minor upgrades.