Years ago a buddy of mine swapped the frame from a 1964 Chevy into his 59 Laurentian. The only difference that I remember was the Pontiac had an extra set of body mounts at the rear. He cut the pads off of the old frame and put them on the 64 frame.
Paul
Yes, 61 to 64 inclusive are the same. It should be fairly easy to come by. There are a number of places online selling them out of the US but you may also want to check with Lothar at Calling All Cars, Douglas, Manitoba http://www.callingallcars.ca/ I bought my donor car from him.