It was updated because the original 47-48 led to support cracking issues on the front sheet metal. To be honest we don't use our trucks like they did then. The best reason to convert is the mid-49 to 1st Series 55 has replacement parts availability. The center shackles and support bumpers are very difficult to source and repair.
https://www.flickr.com/photos/advance-design_parts_co/8129470535/in/album-72157631868452754/
Bumpers are not reproduced, ultra rare NOS.
https://www.flickr.com/photos/advance-design_parts_co/8129510972/in/album-72157631868452754/
Typical rear shackle set and shims, requires drilling and support plates behind seat pan, two holes each side to in frame.