My understanding is that the input shaft is different.

 

There are changes needed. From what I remember, you can get the LT1 T56's pretty cheap, and are sometimes worth the effort of converting them, if you know it's in good condition. Search up "LT1 T56 to LS1 conversion" for details on what you would need.

 

 

The bell housing, input shaft and mid plate need to be changed. I went through it with mine.