Pat I had fits with the strong back too, even after going 3/4 inch plywood. I was out by about 1/2 inch too with the CLC 1/2 inch strong back. Even with 3/4 I was out about 1/8 inch. The following are some tips to help with building the strong back, I know you probably already figured out some of them

1. Let strong back material stabilize to the build environment for few days if possible
2. I used glue and auto finish nailer so I could go faster., no screws
3, I started at one end and built each side until I reach the other end.
4. I placed weights on the strong back while it was laying flat on a concrete floor.
The alignment was still off by about 1/8 inch.

5. I shot a straight center on all four side down the length of the strong back.
6. Cut the strong back to length using the end with less curve horizontally and verticlal.
7. If you have Nicks plans or a CLC kit, there will be for the each station cutout a vertical and horizontal alignment marks.
8. If not make center alignment marks on each stations strong back cutout.
9. Mark location of each station.
10 dry fit each station. If the strong back alignment marks won't align with the station centered marks, trim each station by the necessary amount. I estimate much trim on each station.

Good luck
.