I just sold mine, but I had a magnum 1.08 mounted horizontally in it, bolted right to the firewall. Then I moved the cowl back and forth to get the closest distance to the spinner backplate. I had about 3/8" to 1/2" overlap.
I did have to cut away the cowl so the head would stick out a bit. Could this be your problem, is the head hitting the cowl, and you don't want to cut it?