What I do on stock type cars when putting in
a roll bar is to keep the main hoop out as far
as possible.. this will land on the rocker/sill
plate area.. you still have to put in the 6x6
plates.. I bend that plate to fit the sill.. part
of the hoop will land on top but part will be on
the downward angle.. I cut the hoop to fit that
angle.. then I fit the angled down leg to land on
the frame connector... doing it this way keeps the
main hoop out close to the side of the car to maximize
the interior space and IF you wanted to add more strength
you can add(if needed) short brackets to the B pillar if
it lines up with it... most guys wont do this if its a
street car but if its a race only they gain some strength
doing it.. on a full tube chassis I dont bother being that
the chassis has the strength and the body is just along
for the ride
EDIT
if you put that 6x6 plate on the bottom of the
foot well area.. or anyplace like it.. its just
there t increase the surface area with the 1/8"
material.. its a much greater area to tear out
in a crash/roll over