That is (imho) too big of a gap. Obvious issue is either the floor is installed in the body too low OR the rear section of the frame has drooped over the years. First check the frame a: this happens a lot, and b: the frame is easiest to measure and correct.
Our frame diagram
http://www.wescottsauto.com/Informat...iagram1932.pdf should be a great help. Its not accurate to a gnats eyebrow hair in all respects but has proven to be close enough for people to assemble a bunch of very nice fitting cars.
The key measurement in this case is the droop of the section of the frame from the axle back.
ps. on the frame diagram feel free to copy, use, expand, shrink, give copies to friends at no charge. The only things I ask is do not sell copies, or call it your own. Any errors are MINE!
Karl