Very important and often forgotten. On the 3.8 cars you have to mount the rear bumpers from the inside whereas on the 4.2 cars you have to install the bumpers from the outside. If the fuel tank installation would not be such a nasty and paint scratching job I would probably not mention it. but I think it is fair enough to advice you in order to avoid such hassle and double work.
Here I just finished the installation of rear bumpers on my 885066 FHC. The early cars up to about late mid 62 did have a very slim seal between the bumper and the body. After about mid 62 the seals were wider to allow a less precise and hence more inaccurate body work and fitting. To have such a tiny 1-2 mm gap all along the bumper requires a very skilled work as only a few coach builder are able to perform. Thanks to Rod and Lee Jay, who made such a great work that fitting the bumpers was a pleasure and took me 1 hours to mount both sides.
Here is the result