OK DSwanson had me extremely worried and I went outside and pulled my Miata Seats out of the Mobile Spridget Parts Storage Vehicle( A Chevy Suburban 2500 ) and placed one in the DS of my '68 with a Roll bar. It is possible depending on seat placement to get far more room and seat recline than with stock seats. When Miata seat is positioned properly, front feet of seat 2 1/2 to 3" behind cross member, headrest back will fit between vertical bar on Roll bar and press against the 45 degree brace. When in this position, the seat back of the Miata Seat is resting against the parcel shelf behind the seat. The seat physically cannot recline and further anyway, not due to the rollbar but due to the parcle shelf and seatback. Yes it is a tight fit but in just the trial fit I am much more comfortable than with stock seats.
Welding up a proper set of mounting brackets, and I have a set of blueprints available that adapted Miata Seats to a Flat Floored Spitfire if you want a copy, will need to be done which will raise the seats perhaps 3/4-1" higher off of the floor. Or I may just opt for drilling a new set of mounting holes being careful of brake lines underneath the floorboards.
After seeing a friend's Spridget T-Boned by a Buick that ran a Stop Sign, and the Rollbar that provided side rigidity to allow the Spridget to slide sideways and kept the Buick out of the driver's side,and driver to walk away, I'll keep my rollbar. It may provide a false sense of security but I know I do feel safer because of it.