Comparison of chrome and rubber crossmembers. A stout channel (yellow arrows) lifts the chassis rails on the RB (lower) to give the higher ride height. The red arrows show the difference in the rack mounting brackets: (MG Experience):

According to Jim Gibson also on MGExp whereas the CB rack mounts were the same for both LHD and RHD, on RB they are apparently moved to one side for RHD and the other side for LHD. This makes sense as the shaft has to pass through holes in the chassis brackets for the engine mounts on RB making lateral position of the rack important, whereas it passes under them on CB so lateral position was unimportant. Mark Vaughan in USA posted the above comparison so presumably is showing a LHD RB crossmember, but any offset in the brackets to one side or the other isn't obvious from the picture. However comparing CB to RHD RB there IS a difference - on the RB the brackets seem to have moved about 1/4" towards the passenger side. CB equidistant from the A-arm bracket bolt:

RB offset to the right, although it would be the rack mounting face that would have been positioned by a jig for welding, the bottom of the 'legs' would be welded where they ended up:

This investigation of rack mounts stems from someone on the MGOC forum saying his driver's side track-rod was angled up towards the wheel whereas his passenger side was level as here:

I suggested he take drop measurements from rack, brackets, crossmember and so on to try and see where the discrepancy lay, and he said one bracket was 10mm higher than the other. His rack shaft passes through the hole in the engine mount chassis bracket correctly, so if the crossmember itself is the same distance from the ground both sides it can only be the passenger side rack bracket being higher than the drivers. And that 'IF' is crucial. Despite saying to take the measurements on flat and level ground, and to take multiple measurements, further investigation revealed that it is his garage floor that is out! In other words rack, brackets, crossmember and chassis rails would all have measured the same both sides.

Two upper pads AHH6205 each side with a collar round the bolt-hole that protrudes down into the crossmember, and two lower pads AHH6206 each side that are flat as there is no space left for a corresponding collar to protrude upwards.

Detail from the above showing collared uppers and flat lowers both front and rear.

Rubber bumper and V8: Two upper pads AHH6205 each side with a collar protruding down into the new channel piece, another AHH6205 each side at the front, facing upwards, underneath the original top surface of the crossmember, for the lower pads. Rears staying with flat pads AHH6296 in the lower position each side. Incidentally I'm assuming the V8 should have this arrangement as all V8 had the rubber bumper crossmember. The Parts Catalogue doesn't say so, but then it doesn't say 4-cylinder RBs had the different pad arrangement, until the 77 and later catalogue.

Detail from above confirming that the fronts changed to collared uppers and lowers while the rears stay with collared uppers and flat lowers.