Jim, I just finished a a Trutrak install, wth the roll servo under the left seat. Pretty easy to get to and to inspect on annual. My ribs were a tad closer together that spec so I installed an 032 AL shim under the servo bracket to level things out - sounds like same issue you have. Trutrak supplies a doubler plate for the outside of the belly skin.
All fits well, no interference with manual aileron trim, and first two flights have gone well. Be careful with setback from front spar to servo bracket, if too far forward servo arm bolt will contact aileron pushrod at full forward stick, too far aft and servo pushrod contacts aft edge of rib lightening hole. Just measure carefully