Your totally safe options, in order of investment:
1. Stock
2. CG Lock
3. Seats, Harness Bar, Harnesses with ASM
4. Seats, 4+ point rollbar with a diagonal, 5/6 pt harnesses, helmet, HANS
Safety is a system. And the stock seats are not designed for use with harnesses.
Actual safety requirements for Autocross are generally much less. So you can get away with knockoff harnesses wrapped on the stock cross brace just to plant you in the seat. Not that I would suggest it.
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