You're very welcome Joe, and glad to help. What I was thinking is that the Buck Ajust-tru backplate is quite thick, so that it enters the chuck body deep enough for the chuck's 4 Adjust-tru screws to bear against. This design makes for a very tight coupling with much reduced overhang.
Measuring my 8.25" Buck, entering the chuck that deep means the D1-3 adapter must be about 1.875" thick. Seems to me most "standard" backplates are considerably thinner than that, and may not work for you. I wonder if you may be able to bolt a D1-3 adapter to your existing A1-6 backplate after some modifications?
BTW, congratulations: you're going to love that chuck!