I have a '50 Chevrolet four door that was my late grandfather's. It has a '54 model 235 engine. The body is beyond repair. I have found , locally, a '52 Chevrolet coupe,(Bel air hardtop body with no frame. I'm wondering if the hardtop body will fit on the four door frame? Anybody done it? Thanks.

A quick measurement of the wheelbase on both frames may tell you if that is a viable option. If there is more than a couple of inches difference between them I'd probably say no....

I know the wheelbase is the same on all Chevrolet cars 1949-1952, but I wondered if there might be other factors involved, especially since one, the '50, is a four door sedan and the other, the '52 is a Bel Air hardtop.

Have you looked up the frame dimensions? http://chevy.oldcarmanualproject.com/

It should be the same. I think the convertible may have had a beefier frame and not sure about the hard top. The link Scott posted may give the answer.

In later years, Chrysler (and I'm sure others) added new structure to any non-post chassis (2 & 4 door hard top, convertible). Typically they would reinforce the spring attachment points, and perhaps a frame connector at the rocker boxes.
You'd have to examine a drawing or excellent photo of the bottom of a coupe to tell if any of this is present, and how to add to your frame without disturbing original condition & appearance.