Axle shortening or joining, if it's just an axle, should be fine if it's done well and with a suitable weld process etc, take notes of what you do, what filler rods etc
The exception to this is jag rear ends, this is because the driveshaft is the top suspension arm. These need a more specialized process because a failure would likely be way more badderer
Battery- if your fuel system is modified then the CCM requirements apply, so battery has to be isolated from fuel in a non conductive box.
I have had a customer with a stainless battery box who didn't want to change it, line the inside of the box with rubber sheet so it was no longer conductive between the battery and everything else