The 8.5mm Steering plate is on Back-Order at the moment.
In the meantime: I'm going to try using 3.5mm shims on the Ackerman while still running the Standard 18mm Bell Crank, (never ran more than 2mm) so it will be interesting. This should give somewhat of the same feel as using the 20mm Bell Crank option bits and 1.5mm on the Ackerman shim. I know it won't be the same but will get me in the ballpark for a general feel.
I'll let you guys know what I think in a couple days. I'll have another 2 day weekend at the track (Sat. & Sun.)