The cast iron powerglide converter housings are different and must be changed to move the transmission between a V8 and I6. Here are pics of a '57 235 powerglide and a '57 283 powerglide.

http://s858.photobucket.com/albums/ab147/jdaniel171/?action=view&current=P1010004.jpg

http://s858.photobucket.com/albums/ab147/jdaniel171/?action=view&current=P1010105.jpg

Joe D.