When I fitted my ARB the frame rails had spread outwards. I think I used a couple of ratcheting cargo straps to bring them together enough to get the ARB started. Once it got going it went on fairly easily.
@MANUCHAO and I (mostly him) did the same things. Get some beefy ratchet straps, a sledgehammer and a big block of wood(to pound the bumper in once you get a little bit of it on. Use the block to protect your bumper)