7.8 and 6.6 what's interchangeable from one to the other

Hi everyone I'm sure this has been answered many times I have looked and didn't find any info. I have a l8000 dump with a 7.8 my hats been sitting and I just recently picked up a f700 with what I thought was a 7.8 but it is a 6.6 instead I am wondering what I interchangeable from one to the other?? Are the turbos the same also
 
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