Anyone done it? I have access to 2 low hour IT175s. I may swap over the rear shock from one to my 80 DT175 while mine is out for rebuild and service. Was wondering since the IT175 is so much faster than a stock DT because of porting, and a little higher compression will the entire top end swap out? 

Check out the base gasket comparison online is a pretty good indication of what fits what before going further.

Neither the shock or the complete top end of an IT175 will fit your DT175.

The whole 77-79 IT175 engine will bolt in, no autolube or tach drive though and carb and pipe are very different (DT parts can be made to work.  You'll also need to swap the countershaft sprocket and spacer to the DT parts)