You should not have to do anything with the differential flanges, you should already have the 10mm non cv style flanges on your shafts at the slip end. So both diff flanges should work as is. If you are going to run high pinion with dual cases you should not need a cv in the front at all. I never ran one with that setup and Experienced zero vibrations, you might have different results but if your truck isn’t lifted 15” you should be fine.
keep in mind, some instances a stock/clearanced cv will need a limit strap so it doesn’t over travel the cv on suspension droop. the non cv shafts get more angle (84’+) the early stuff gets less angle, either way should not be an issue with duals and a high pinion. That is more for single cases/low pinion trucks that are lifted. I only bring this up because a lot of people think they need a cv front and that generally isn’t the case.
I would seriously consider running a long slip front shaft with no cv. And running a clearanced cv rear shaft. Either drill the pattern into your t-case flange you have, or put the original front case flange on the back so you can bolt that cv up to it.
