You'll gain even more from relocating the battery if its as far back as you can get. So not the high up near the rear seat back but even further back by the bumper. If a 20lb battery is placed say 2ft behind the rear axle, it will create a moment about the axle and is like removing an additional 5lb right off the Front axle of a 3600lb B-body.
That thinking is adequate for straight line racing. Putting the battery far back is not ideal in racing involving turns. The farther you have a weight from the center the more it wants to resist turning. That is why a mid engined car out handles a rear engined one, all else being equal. Under the rear seat pass side for the battery, ala VW Bug, is ideal, but likely no room.
