Phillip:

If you're using a 1 1/2" x 3 1/2" box beam, make the spacers out of 3/4" x 3 1/2" material. If you have 12" spacing w/ 1/2" thick forms, make the spacers 11 1/4 inches long. Cut one end at 90 deg, the other end slopes from the 11 1/4" to 10 3/4" on the other side. Now cut some 5 1/2" long wedges that slope from 5/8" on the small end to 1 15/32" on the long end. Yer done.

Put the right angled end on the side of the strongback at the form line w/ 2 screws, same for other side. Slide on a sloppy form. Place the next 2 spacers. Etc etc.etc.. Then every thing is straight forward.

-mick