I was also advised race harness's are not approved because they are not "idiot proof" ie you cant just buckle it up and it self adjusts to suit differing people. In a harness you need to manually adjust them.
And as Dimis has said, dont ignore a second hand car that is approved (in your State) and rebuild it to suit your needs, it may be a higher initial purchase price, but does get you into one quicker and saves the hassles of engineering approval.
You may be able to twist my arm and get mine
Then I will build a Daytona with a roof and air con
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Cruising in 5th
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Never be afraid to do something new, Remember, Amateurs built the Ark: Professionals built the Titanic.