First let me ask you the million dollar question...how much are you willing to spend????
Dynacorn complete floor $3000, wheel houses $170 each or $340 for two, outer rocker x 2 $120, quarter panels, $260 for mediocre pieces and $500 for OEM quality, $150 for rear tail panel and rear valance,so you are now at between $3,880 and $4.110 just for the parts mentioned above and this is just the tip of the iceberg...IMHO
My guess to totally restore your car is $15k and that is you doing most of the work yourself including body work/prep before you put a $3/4k paint job on it.
Here is a car I have seen in person and know some of the history...it's not perfect by any means although it is a 289/factory 4spd car and truthfully worth $18/20k in my book. I'm not sure how much you paid for your Mustang and after all is said and done you will have around $15k plus whatever you paid for it...if you paid more than $5k you will be upside down in the car...IMHO