Nah, it's #15 rubber grommet that needs to be factory correct if you are looking for a concours winner.

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ex-NSW Police 1970 MK II Cooper S
VMCI #43

Does anyone have a measurement for where the bolt (#8) for the bottom of the strap should be on the floor? When I mock mine up it seems like it should be inside the sealed area of the beaver panel? Doesn't seem right as I was expecting it to be further inboard where it was accessible from underneath?

My 4/66 S does not have a captive nut below the floor like later Ss - just a hole, but the bolt is just accessible for a nut/washer from under the beaver panel and is very close to the panel that seals the corner of the beaver.
I can give you measurements if I can work out any reference points. It's the same location as the later ones with captive nut.
Bill

Well I bit the bullet and drilled today. The hole is referenced from the left tank and is as close as I could get it to the panel on the beaver. It still doesn't seem to be far enough in when I have the tank fitted from what i have seen from other pictures of twin tank boots but I can live with it.

I spent about 3 weeks trying to get the RH tank to sit with a decent amount of filler neck sticking out of the body like LH. The original hole in the floor I found is slightly too far towards the centre of my car causing the strap to not let the tank sit towards the outside. I elongated the hole on the strap end to get more movement.
Your photo seems to show a reasonable amount of filler neck, even though your tank is slightly towards the middle of the car compared to where mine ended up.