There isn't much as far as hard parts that interchange , and your 518 will have many of the most up to date parts . you can put 5 clutches in a 4 clutch drum so there is no reason to add the rotating MASS of the $$$ hemi drum and it's rear clutch retainer to the 518.

Taking and upgrading to the 618/47RH would be more worthwile , there isn't a 6 clutch drum except for maybe in the 48RE ??? , upgrade to the diesel/V10 OD rear section , that has more clutches in its setup , I think it's the same outside hard parts just thiner steels and clutches , but not sure haven't compared them side by side . The diesel trans has a 5 pinion steel front planet , make sure the spline count is the same as your trans .

Then there is lockup vs. non lockup and adding the controls to turn off OD and apply lockup if you go that way ...