Diesel Kleen is another fluid that will eat carbon like a fat kid eats cake! It's cheap, nd whatever is left over you can just dump in the tanks and burn it. It will help clean out the fuel system.
I would think that applying some antiseize to the new injector threads would help the threads fron seizing, but the only way to keep the soot from binding them up from the inside is to ream the seats, and use new copper gaskets (washers).
Do you have an injecto install kit already? If not, Typ4 (Russ) sells the diapco kit (supposed to be the BEST on teh market) as well as viton Orings. I have a set of rings from him as well as one of the install kits. Great guy to deal with and his prices were better than ANYTHING I could find locally...and he shipped them from his home in Oregon...I am on the east coast of Canada!