When you say you've "...already got brackets...", are you referring to the pad abutment brackets that cradle the pads and caliper or the backing plates that bolt to the axle housing, contain the parking brake components, and to which the calipers are bolted?
Maybe I don't understand your question, but if you have the backing plates the originally bolted to the 10-bolt S-Truck housing, then just use that as a basis to make your new brackets. That should give you all of the critical dimensions with respect to the radial dimensions. The brake offset dimension is different between the 10-bolt and 9", so you'll need to take that into consideration for axial positioning, but bracket design could vary depending on whether you're thinking weld-on or bolt-on.