Well the main reason, this is an example. Your T code coupe, butchered into a fastback will never fetch the value of my REAL A code fastback, all original metal, minus the hood. And I would imagine as time goes on your butchered car might have some corrosion problems from the dissimilar metals welded together. This is just a hunch.
At a real mustang car get together, where there are more purists than the 'conversion' people. Some of the attendees might say, cool car, too bad its a Fake. This is just my belief from the people I know with these cars.
This 'Eleanor' thing surely put the fastback craze into people. But the original, period correct fastback is the coolest car on the road. These kids with all the stupid large, 17 and 18 inch wheels and plastic all over, just ruin them. And when I look at your conversion alien thing, I will immediately find the telltale sign of a phony. I understand this is a free country. But if you want to ruin a car with this crap, go get a POS chevy.
And by the way, a fastback cougar?, whatever.