The best place is as close to the middle of the pivot and as low as possible. So center of the axleleft to right, front to back.
In my 2wd, it sits under the seat - works really well and is super stable.
WIth i2c extender you can go a few meters with i2c.
On the 4WD it is mounted right at the articulation joint.
you want the imu to not exhibit any non symmetric motion - different going left vs right - by having an offset. Move the thing around the cab, or even on top of the cab away from blower motors, high power etc. The ability to mount the thing away from the confines of the autosteer board is key as you can easily mount it pointing forward using a single axis.