Yeah, I was running 1-1/8" thick spacers all around. The wheels (both 6" and 7") were dished to the outside and the spacers got them out to the fender lip where they looked "right". The rear was much closer to the fender than the front.
You can do the same thing Dave suggests by setting a 18"/24" carpenter's square against the part of the wheel hub where the rim attaches, then go up to the point equal to where the fender lip is and measure that horizontal distance across. That's the max space you have for wheel + tire width (outside of the tire) to the inside of the fender, then subtract another 1/4" - 3/8" for "running space".
You can measure how far the tire on your DD sticks out beyond the rim - Just place the carpenter's square across the wheel and measure the distance to the rim lip and I'll bet it'll be about the same on your new wheels and tires. Alternatively, you could mount your chosen tire on a similar width rim to see what the outside tire "pooch" depth is. Subtract that amount from the distance you got just above, of the max space you have for wheel + tire width and add another 1/4" - 3/8" for running space. That is where the outer rim lip has to be, measured from where the rim mounts to the hub/drum.
Next, measure from the hub/drum inward to the outside surface of the spring plate and subtract the same amount of tire width you got from your DD, then subtract another 1/4" - 3/8" (your choice) as "running space".
Take or email all those measurements to your wheel builder and they can build a set of wheels to fit your car.