If you want a rollbar for looks. You can do what ever you want. If you want it to pass tech at the track. You will have to meet NHRA specifications.
NHRA (super pro, pro, sportsman) Roll bar mandatory in all cars running 11.99 to 9.99 with stock floor. If you can't run these numbers you don't need a roll bar. I was told that 2005 rules will change bar to 11.50 to 9.99.
NHRA Gen Reg 4:10 Cars without frame use 6" square 1/8" steel plates on top and bottom of floor, securely bolted together with at least four 3/8" bolts, or top plate welded to rocker sill.
There are other regs regarding height, angle, placement, diagnols, welding, etc. I would take the car to someone familiar with installing bars and who is familiar with NHRA regs.