From my experience fitting mine, now the PTSD has subsided.
I had to get a rubber mallet and bray the wheel arch down about 15mm cause the seatbelt bolts were binding on wheel arch and subsequently it was wonky. Felt horrid.
First attempt I didn't have the bed far back enough to allow my fridge cupboard door to open... i only focused on it not touching the rear door when opened out and having an even gap along side the unit. This was obviously an error.
In this position I then drilled rear holes and bolted in the rear, with strengthening bar - spare wheel carrier moved to the lower hinge point of the two to allow for the spare wheel to go back on. Seemed all good. Not ideal about the fridge cupboard though when I realised but I felt I'd already committed. Meh
Then... when I drilled the front holes out i realised that the bolts came out underneath pretty much against the strengthening struts. So enough room for the bolts to go through but tricky to get a nut on and no chance of getting the 6mm plate square reinforcer washers on. Too far forward...
So I accepted I needed to reposition and redrill so unbolted the rears and pushed rib towards the rear door (in bed layout) so it was pressing gently against door (more than just touching, but less than deforming the cushion if that makes sense)
Again, once there I got it alligned against the unit, i drilled it out and the rear bolts fitted and the rib front left and right bokts were better. although the front middle bolt is still pending as it's in an absurd place to reach underneath.
I'd say make sure you've brayed the wheel arch out where the seat belt bolts hit it and push the rib back against the rear door gently and then check allignment down the side units is even.
Good luck!