The e46 is know to have issue with the floors cracking or ripping out so it’s best to add some reinforcements. We went with the plates from redish Motorsports out of the UK. These are nothing short of amazing. REALLY well designed.
Start by cleaning all the undercoating off.
Smooth out and put etching bare metal prep on them.
POR and seem sealer
Rear subframe also got new poly bushings
subframe back in