Tim, a pretty simple list to remedy, so you must be happy with that.
The rack stops are easy to remove - just pop inner boot clips and then hit the plastic collars with a chisel or similar. Be wary of the inside edge of the rim hitting the leading edge of the lower arm on full lock. The lock even with the collars removed is pretty limp for the wheelbase.
If the lambda is out, I'd doubt the cat is at fault. What calibration are you running on the ECU? Is it possible the ecu is maxing out fuel trim, and unable to dither the fueling about stoichiometric? What's your fuel rail pressure?
I suspect the wheels you have are 7" et35. Might be worth borrowing some stock Rover wheels to make life easier - I think they're typically 5.5" or 6" depending on what car they're from, so may enable you to comply without bodywork modifications.
IVA - good use of half term.
