I used to work in a chassis shop. I had no bar in my '66 and asked the owner about mounting a harness. His one question was "why would you want a harness to hold you upright when the roof could come down and crush you if you go over?".

You are safer with a stock seat and lap belt.

I gave up on the '66 when it was going 12.0's at 110 w/o a bar. I was still legal, but somewhere along the line common sense has to kick in.

The plastic seat will not pass tech without a brace from the back to the rear floor anyway.

Get an NHRA rule book. It has very specific instructions for seat/belt mounting. Yes, it's $10 or so, but it is money well spent.