70 Duster power to manual brake conversion

Hi guys,

So I tried to convert my power brakes to manual, and I thought it was going to be at least somewhat straightforward. Of course I was wrong, or did something wrong. I bought a new master cylinder and adjustable brake rod from Doctor Diff, which is listed as "Super duty heim-joint style master cylinder push rod adjusts in length from 6.1" to 6.8" (center of bolt to end of tip). Fits '66-'76 A,B and E bodies equipped with manual brakes.". Well, mine wasn't equipped with manual brakes so I guess that's where the problem starts. Since I had read of people just using a stock manual brake rod, I figured using this would make sense, especially since it is adjustable. I removed the booster and put everything back together, and the rod is at least an inch too long at its shortest adjustment. There's no room for any more padding or adapters at the firewall... the bolts won't handle any more and still be able to have nuts put on them. I tried to put a plastic plate piece from the old booster setup in there and it still wasn't enough. So what am I doing wrong here? If this rod is for a manual brake 70 duster then wouldn't one of its lengths match what I need?