Body Work, or New Car?

Paint is easy. Body work a bit harder. Replacing interior panels even harder. The biggest question is how good are you at bodywork and how good are you at turning a wrench? From the looks of it you will have about the same money tied up in it either way. If you lean toward the wrench turner side....do the swap. If your good at body and have a resource to cut panels from....go for it. The painting portion is easy depending on how you go about it. If the 4cyl just needs a new coat, sand it out seal it again, spray color and clear the snot out of it. A lot of the newer paints will draw trash to the surface and then through the clear when sprayed. The vast majority of this can be wet sanded out if the clear is thick enough. Look up "body work" and youll see the front end of my car. All of the portion that has been painted was done in my dads garage (no air dryer, paint booth, plastic drops or pressure adjustments on the lines). Trust me, It could have been done just as well on a dirt road. Might be dirty in the pics (due to the rest of the body work), but have even been told by an auto body friend that the front came out well.
The short of it is...what are you better at, whats cheaper, and will you miss your current stang?

I'd say go with the new car. I havnt looked into the difficulty of the 4-8 swap, but would think that you have all the parts from your current 5.0 to do it. If the body shop does all the repairs on your car it will cost big time. The apron would be my biggest concern. The rest could be done fairly easy. The only reason I try to do my own is that I have always wanted to say "I did all the work". Every car I have had, I would have to say "everything, but the paint or motor". Besides I have found that you get what you pay for when it comes to auto body, and damn good work is expensive.