A few random points.
I bolted my seat to the floor pan but i used strips of metal on the underside of the floor to bolt the seat to and i even welded the bars to some supports that were on the floor so you can fabricate back mounting that will be strong.
I had drago install his tunnel pan which gives you more room to move the seat loser to the floor and tunnel.
Lastly, in both my cars, two different cage builders the role bar on the top driver side is inside the seat and my helmet hits it. but with some padding on it, its a great place to rest your head on right handers. In other words there is nothing that can be done about that other than race something other than a Miata.
I spent about 12 hrs installing my seat and making modifications to make it perfect, you just have to put your time into getting it right.