If you're concerned with head clearance, give the Ace a look - I went with the ACE AC2D-H (doesn't work with glass rear window with the -H option, but provides better geometry for shoulder harnesses) and added door bars. From a stiffness perspective I don't think anyone would be able to tell the difference. The Ace model moves the down legs of the front tubes and ties in to the b-pillar with a tab and everything else clamps to the tub in the tunnels next to the fuel tank. The forward most part of the whole assembly is several inches behind the back of the seats, and it doesn't extend as far as the door jamb so there's nothing for a flailing head to make contact with. The Ace bar carries the same certs for NASA and SCCA provided it has the diagonals.
The pic on the bottom right is the one I have in my 90. http://www.bethania-garage.com/ace.htm
Chris