I have a RE 5.5 long arm kit and I want to extend my wheelbase six inches do I need longer/custom arms in the rear to do this? Or can I get away with the ones I have?

you will have to relocate the mounts for the arms in order to push the arms back or forward. easiest way is to buy a longer rear drive shaft, weld the mounts 4 inches back and cut the rear fenders.

The rear axle on my 98 would have required me to change my
fuel tank to move it more than an inch back. Not to mention the
control arms - brackets - arms - etc.

I was thinking about choping mine to get ten more inches in but
decided that the change was cost prohibative - and I'm getting
a 2006 Unlimited body and frame that already has the added length.