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Get clarification from the organizing club you plan to run with and if you change belts, be sure to do it exactly as described in the harness instructions. Most clubs require a seat with pass-throughs for shoulder, lap, and sub belts. I don't see these from your pic but may be there. Every organization handles safety a little differently so go directly to the local source for specifics.

From my perspective, a seat, harness, hans, and roll bar ext are a package deal and I feel less secure in a harness only than in a stock 3pt in a totally stock car. If you plan to add a harness, do it all at once as a complete safety system.