I was hesitant about replying to this thread ,but i have something that might help.
I know you`ve seen the thread on "The Tool"
Well I don`t have machine shop capabilities at home so...
I took a 1/2in. coupling and on a bench grinder sharpened it to a point.
A 1in. coupling I cleaned the threads out and kind of rounded off with a dremmel tool. A piece of 5/8 all thread and a couple flat washers and made this.
I drilled a 7/8 hole in the keg ,put the tapered coupling inside ,the 1in. coupling on the outside and (after coating everything with 3 in 1 oil pulled the coupling through the hole.
Then switched out for the new coupling and pulled it through the hole.
I ended up with this. should be real easy to solder.
of course i clean every thing with denatured alcohol.
Disclaimer : The 1in. coupling was galvanized,and the keg was a test piece ,I will find a stainless 1in. coupling before i do the final.