Ridgid and Milwaukee power tools are both made by TTI and share many of the same parts. Just by looking at some pictures, the Milwaukee 42-06-0650 1/2" Square Drive Anvil Assembly looks very similar to the 42-06-0025 3/8 in. Square Anvil that I used on mine.
As for the Stealth Force Brushless...
Well I got the 3/8" Square Anvil and I can confirm that this Mod will work on the GEN5X 1/4" Brushless Impact Driver (R86037) as well. There is definitely a noticeable increase in the amount of torque this puts out. The 3/8" Anvil hits way harder than using a 1/4" Hex with a 3/8" adaptor.
I was wondering the the same thing. Has anyone else attempted this mod, and how is the OP's holding up? I just ordered the Milwaukee 42-06-0025 3/8 in. Square Anvil and i'm going to attempt this on my GEN5X 1/4 in. Brushless Impact Driver (R86037). I'll post updates when I get the part.