It looks like your on the right track, the slots in the floor are in the right place, although you will need to run them further up the diff hump, basically to the original cardboard firwall line[;^)] This will allow the rear end to go nice and low[:p] You also need to make sure you can achieve antisquat geometry with the bars, that will dictate your floor box mounting heights

Without antisquat, you might as well just add tramp bars