The TR6060 is actually a very rebuildable unit and a competent transmission specialist with manual gearbox experience does not need to be a Chevy dealer to do this job properly, the Tremec design is well documented and the synchro replacement is straightforward for someone who knows what they are doing with performance manual gearboxes. What you want to avoid is a shop that pulls the box, replaces only the third gear synchro rings, and puts it back together without inspecting the other synchros and shift forks for wear while everything is already apart. If the previous owner was genuinely hard on this gearbox the damage rarely stops at just the one gear that is currently presenting symptoms. Ask any shop you consider specifically what their inspection process covers while the box is open and how they handle finding additional wear beyond the presenting fault.