I recently modified my 2853-20 to use a 1/2" anvil since I was tired of breaking socket adapters and having the collet drop bits occasionally. I came across this post and thought I'd share a picture of my custom M18 Stubby Impact.
I used an anvil and the bearing that sits behind it from a 2755B. The internal dimensions lined up perfectly, however, I had to bore the nose bushing larger using a 21/32" drill bit with some light reaming. It is very powerful and feels just as strong as my 3/8 M12 stubby. I don't currently have a way to test how much nut-busting torque it actually has but it rips off lug nuts torqued to 140 ft/lb with ease.
Although I am very happy with the outcome, it's important to realize that this type of modification will void the warranty, etc, etc.