That is a lot of shim thickness. I would be afraid of that restricting fluid flow back from the converter. Or maybe to the converter. Not sure. But either way, the easiest thing to do is use the old Magnum flex plate to mark the position of the weight onto the converter itself. Then remove the weight from the stock Magnum flex plate and weld it onto the converter. Now you can run any neutral flex plate that fits your six bolt crank pattern and four bolt converter pattern. This is basically what the old Mopar weight kit was for, but that is long discontinued.
I had a post in another thread showing how to mark a neutral B&M 10230 into a Magnum 10242 but that one also has the larger converter bolt pattern that interferes with some 904 setups because of the diameter of the 10230.
I should also say that when making any real power over a stock 5.9 Magnum, a better flex plate should be used anyway.