It might be worh considering a few options to speed up the healing process if you are going to remove such a large branch.
1. You could try the technique where you cut their the branch had way, leave it in for a year or two to encourage some h&r pealing before removing the rest. This starts a callous on the wound, and should leave you with less to heal once you cut the rest of the way through...
2. I have seen people describe a method where your leave a flap of bark below (I only read about this a few times and cannot actually recall the full technique, but I think it is in Naka's first book) that is large enough to cover most/ all the wound, then tie the bark down to hold it over the wound until it fuses... I was thinking this might be a good one to try on an Arakawa so that you do not wind up with a large swath of non- corky bark right in the middle of all that gorgeous stuff... Just an idea I have read about and never tried myself.
Regardless, his tree has the bones TJ be a very nice, LARGE bonsai. I hope you turn it into a masterpiece and keep us all well informed of it's progress!