Take a tape measure along the outside of the fuselage from the leading edge of the wing to the firewall ( or wherever the center of your battery lies) and subtract that measurement in inches from 70 and then you've got the "arm" value you need to calculate the W&B adjustment for your new battery. I'm about to do the same for my RV6's new battery.
edit: Google for the "Aircraft Weight and Balance Handbook" by the FAA for a pdf of a good booklet for doing W&B calculations. The knowledge in that book works equally well south of the equator too :-D