just an FYI ... my 58 and 61 wagons Canadian cars also has 1 piece bumper, on the 58 Delray delivery both bumpers are 1 piece, but on the 61 Brookwood front is 1 piece and 3 piece on the rear (maybe because is a 9 pass?)
I can't speak to the Canadian cars using one piece bumpers, but the CA cars were required by State law to have one piece bumpers. As previously mentioned, it was a safety issue for CA.
The one piece bumpers used less brackets also which in my mind made them lighter which is why I think you seen them used on a lot of the drag race cars,less brackets and less bolts
I have walked a few bone yards up here in Canada, and it seems that many if not all the Canadian built 61 and 62 Chev's had 1 piece "California" bumpers, front and back. My theory had always been that they were cheaper, and that the salt would probably not have been a good thing in those joints in the bumpers.