Did you leave the bolts loose where it attaches to the frame? After you run it through the floor can you clamp it in under the dash (with the rubber isolator in place)? If the cab is set to spec and it still does not fit run a straight edge along the column, is it bent? This does happen sometimes. When everything is clamped in place it is a pretty rigid set-up who knows what might have happened in the last 60+ years. The horn wire should come out on top as pictured not to the side where a knee could hit it.
