Discovery rear cross member replacement

Looking for any advice/tips/guidance on replacing the rear cross member on a
91' Disco. I've got the bumper off and it's all rotten. Looks like I need an
LRD157 (cross member) and two LRD158s (end pieces). I'm guessing the fuel
tank needs to come out for welding access. Do I need to support the body
while I cut the rot out? Any advice, horror stories, photos etc. gratefully
received.

Thanks in advance.
Peter