If your question is will these headers fit, the answer is... may-be.
The trouble is that all tube steel headers are basically hand made, using jigs and fixtures, there is always some variation to deal with. Granted, a good initial design is key to reducing the fit problems introduced by production varations and some of the Clifford sets were well designed.
(The 60-66 3 X 2 were not, at least the ones that dropped the front 3 tubes through the K-frame legs and low under the car)
Here is a better question, can you even purchase a set anymore? Last word I had is that Clifford had dropped many of the long tube 3 X 2 headers, seems like the "shorties" are the only ones they still offer.
DD