If your antenna is 3 m from ground 1 degree of rotation will be ~5.2cm. So if your cab tilt 1 degree more than tractor you will have error of ~5cm.
If IMU is mounted on tractor reading 0 tilt and cab with antenna is 1 deg side ways it will drive ~5cm error.
IMU on tractor antenna on hood fixed to tractor eliminates cab suspensions error.
Or IMU and antenna in and on cab with IMU corecting for cab tilt. It shoud be better to leave IMU in cab. Better to mount IMU lower than higher.