There's a lot more to it than most people think. For one, the roof height at the windshield from wagon/sedan to coupe is as I recall, over an inch difference. The wagon/sedan windshield is not laid back as far as the coupe. Then there's the whole curved side glass issue (I have a guy that says he can get it made).
I would absolutely NOT put flat glass in it, there's a welded-rear-doors wagon for sale here in OK done into a sedan delivery...the area immediately behind the door glass looks terrible...almost flush in the middle, with deep recesses at the top and bottom.
IMO, best bet is to have a coupe donor, use the doors and B pillars from the coupe, and the forward portion of the quarters, sectioning in the wheel opening. I would leave the roof alone, and use the wagon door uppers to build a window frame for the coupe doors.
I had both a coupe and wagon for the project, but sold the coupe (too good a driver to whack up), and have not yet found another reasonably-priced coupe to use as a donor.
There has been one for sale in TX that was spliced as suggested in the post above, but the roof had a hump in it because of the height difference...killed the flow of the roofline, and made it look amateurish, despite supposedly having been done by a long time bodyman.