It seems like you should able to cut out most of the old battery box and replace it but I really would consider a fender pan replacement for two reasons.
It's not that big of a job to pull the fender pan off and secondly (at lest around my area) those pans are going for cheap $$. I have removed the battery boxes off a couple of older narrow frames (70 and 100) which was a fairly easy job thanks to spot welds. Seems the guys who had them before me decided to expand the box for a bigger car battery
What ever you decide
