Numbers Matching

Ask yourself what you want the car to be. What is your vision for the car? Then go build that car. There are more than enough original cars in museums. These cars have many short comings by todays standards such as handling, brakes, safety, comfort, etc... As nice as it is, I see a car like yours as a blank canvas to build on.

Do whatever you want as long as you do not swap a Chevy LS engine into it!!!! lol

I have a "A" code fastback & had the same dilemma years ago. I went with usual engine mods (Edelbrock Intake, holley carb, pertronix coil, etc).
Tri-y headers & Koni shocks for suspension. I got rear replacement interior panels to cut up for speakers but didn't butcher the dash for a better radio.

My thinking was If I kept all my original parts then I could always go back to that but still enjoy my car like I want it to be. Anyways that was 15 years ago & still happy with my decision today.

Your car looks great & I'm with mustangbrad stay away from a Chevy Engine.

My fastback was a low mileage, one owner, C-code, C4 car with nearly everything original and well cared for. I gave the stock engine to my dad for his street rod and sold the trans for $75 to a friend. My thinking was the car was one of a zillion, there are actually a ton of "numbers matching" Mustangs out there and it wasn't anything special. If it were a Boss 429, or Shelby I might think differently. But it wasn't and I didn't.

build the car YOU want,as the others have said. unless you are looking to try and make money off this car, which is hard to do anyway, a few well placed upgrades wont hurt anything.

drum brakes all around? upgrade to front discs, they were factory available, and the factory style front discs are inexpensive enough these days t put them on.

want more power? cam, carb, intake, heads, and exhaust mods are all reversible if necessary, assuming you decide to sell the car later on, and you can always save the stock parts so the new owner can install them or not.

suspension upgrades, chassis upgrades, tires and wheels, again all reversible by the new owner.

in the end there are tons of C code mustangs out there, and many of them are either survivor cars, or highly restored cars. so again i say build what you want and dont look back.

Unless you have a "K" code there is no such thing, in the Ford world before the '68 MY, as "numbers matching". The "provenance" of the engine and transmission being original to the car can only be estimated based on the casting and assembly dates being within the "normally accepted" date range prior to the scheduled build date. For instance, if the engine assembly date is AFTER the scheduled build date there is an almost certainty that it's not original to the chassis. Because the scheduled build date can vary from the actual build date there needs to be some leeway here, but if the assembly date is 2 weeks after the scheduled build date it can be said, with 99.99 certainty that the car didn't leave the plant with that engine. The same can be said for an engine assembly date that's six months prior to the scheduled build date... Ford didn't stockpile engines that far in advance. FWIW, "Numbers Matching" is more of a Chevy and Pontiac "thing".