1. If that tape measure in your picture shows the length of the existing radiator, it is 4' 2 1/4" long, not 5' 2". Equivalent to 1,276 mm.
If you got a 1200 wide, you would need to "make up" 76 mm. One 1" and one 2" extension = 76.2 mm, so should do the job. Not symmetrical but you might be...