On a 66 is the 4 door sedan the same as 2 door hardtop. I did a search and only found old posts about 68-72 on this subject.
I appreciate any info.............

Definitely the 68-72 frames are different because the wheelbase is different by 4 inches. However, I think that the earlier models are the same. Try a Google search on Chevelle interchange and get to the website on anglefire that has interchange chassis info. That could be a big help if no one here can give you the info. Be patient, as many well informedfolks visit here.

I found that interchange sight but it was not specific about this all it said as some cars have different mounting points but did not specifie.

According to Hollander, 64-66 2-dr sedans, 2-dr hardopts, and 4-dr sedans; and 66 4-dr hardtops have interchangeable framse. So the answer to your question is yes.

Thanks, Does it say anything about any mount modifications. I have noticed lots of times when hollender says somthing is the same they
will list footnotes about modifications needed. I have a guy interested in buying one from me. I just want to absolutly certain that I give him accurate information...........

There were no footnotes listed in the Hollander book (64-72 Chevelle version). The one thing which comes to mind are the provisions for clutch and/or shift linkage which were welded onto the frame by the factory only as needed. But that is not a show stopper.