I did the same with Paddocks, was fine also ordered the drop arm as it saved taking the old one off, its a sod to get off the old box.
Putting it back in is a two man job as the bleeder weights about 20Kgs and trying to support the weight while locating the bolts is a bit of hard work.
One particular pain was getting the fluid feed pipe back in it took me the best part of an hour as the pipe is very difficult to get lined up straight so that the threads catch.
Good luck a couple of hours should see it done.