I have a 1999 c5 corvette it's automatic and I'm looking to start racing it. But should I convert it to a 6 speed or not what's better for circuit racing such as SCCA racing. Also how hard would it be to convert it and what would I need. Just the transmission?

 

It's a VERY difficult,complicated and expensive swap. You need all the peripherals for the manual. Clutch pedal and linkage,computer and associated wiring loom,plus wiring loom for the safety and cruise control switches on the clutch linkage.shifting linkage with switches and wiring associated with that. The best way to do this swap is have the car with ALL the parts sitting next to yours,then transfer one part at a time.
It would be ultimately cheaper and more protective of your sanity to make a trade or sell your car outright and buy a six speed car.
I seriously doubt that you could be competitive for very long on a road course with an automatic car,I don't think the brakes would allow it. Or if you shifted the auto down manually for every corner the tranny would not allow it!
As a test,change the brake light switch on your C5 and then multiply that by a factor of several hundred and decide if you want to convert!