I also agree on sticking to a '65 or '66 Frame.
Also Convertible Frames are so hard to find, it would pretty much be a waste to put on under a Hard Top.
If for some reason, you don't go with the '70 Convertible Frame, let us know if you would as it would be interesting to check into it.
I have some '65 and '66 Frames here and I'm not that far from you, but I don't have any ready to go at this time. They are presently all still under the cars.
-Parting Out over 75 '58 to '73 Full Size Chevy Cars-
-National Impala Association
-Vintage Chevrolet Club of America