With the wide ratio kit already installed there shouldn't be any issues rebuilding the transmission, it's pretty straight forward, and just the usual check of condition of the components should be good. I wouldn't expect any issues with the parts or clearances, assuming things were good when last assembled, with that amount of use, but always good to check.
When I was bracket racing more often I asked the convertor company how often I should send my convertor in to be freshened, and they recommended about every 300 passes, with my combo - a 3400 lb car running 11.50-12.00 ET's and foot braking. The one time I tried stretching it out until the end of a season ( since I hadn't been having any issues ), about 400 passes, I had a convertor failure, so I went back to adhering to their advice. I know lots of people that rarely ever send their convertor in, and seem to get lucky, but that never seems to be me. And some of those people never seem to win either. When you are trying to be competitive and consistent I always found keeping the stuff maintained helps. At the same time I would send the convertor in to be freshened I would freshen the C6 with new seals, convertor bushing, band, Sealing rings, and at least new Reverse-High clutches. The Reverse-High clutches & steels were usually the only thing showing wear and heat issues, the governor distributor sealing rings also usually showed wear. Everything else got inspected and replaced as needed. Occasionally I would have to replace a one-way clutch, planetary, drum, etc. the seals always looked good, but they are cheap to replace, and they take some abuse from heat. I eventually built two identical C6's and had two identical convertors, so I would just swap each time and rebuild without having to be rushed.
There are a few places in the cities most of the racers seem to use, but I've always done my own, so I don't have any first hand recommendation.
*Edited to add: I usually run a 4500 stall 10" race convertor.