The main difference is the rear overhang on the turret which is lower on the low bustle turret. Not only will the turret roof need to be angled down, but the floor of the overhang will need to be lowered as well. Other than that, the gunner's periscope on the the low bustle turret usually have the standard filp-up tin cover with a wire periscope guard, whereas the high bustle turrets usually have the more solid armored box periscope guard. The low bustle turrets were often fitted with the later wide mantlet, so you don't have to change that. You can, but do not have to add the applique cheek armor. You can find plently of examples of Shermans in which the hull was fitted with the extra armor, but the turret was not. Good luck.
Here are a few links that I have used in the past
http://www.onwar.com/tanks/usa/index.htm
http://www.jedsite.info/tanks/sierra/sherman_series/sherman-series.html
http://mailer.fsu.edu/~akirk/tanks/
http://www.wwiivehicles.com/html/usa/m4_sherman.html
Here's a cutaway profile.
