Mmmm... i know what you mean... Fastbacks are fastbacks and i besides it is my favorite body style in mustangs, the job you are planning is complicated...
I´ve seen 66 - 70 mustangs coupes to fastbacks in the net... it can be done at least in those years.. in fact, there is people who sells the body kits to do that fot about 5 K
But in our body style mustangs, i dont think the transformation worth the money.. for less than 5000 dollars you get a decent ORIGINAL mach 1 and you still can have the coupé...
If the shell you found is decent, i´ll consider the chance of using it as the base of the new mustang and just put the coupe parts in the fastback shell... You are gonna need a bunch of parts that are different... Back glass is obvious but interior has a lot of differences too... roof headline, back seat, quarters and all the back part of the interior trim... Don´t know.. if you start counting on new parts you´re gonna need, i´m pretty sure you´re gonna get close to the value of an actual restorable fastback... Think it through and i´m supportive on you buying another mustang, this time, a mach 1 !
Bottom line, if you want a fastback, GET ONE... you´re not gonna regret it