Hi Phil,
I think it was only the early 500 series cars that didn't have the slot already in the tub (unless someone can clarify) I've seen 3 cars plus mine that has the slot cut to allow fitment of the oval backing plate., Some foam filled with black gaffa (duct) tape over the hole from factory. To fit my plate you need to dig out some expanded foam and cut a smaller slot (letterbox) to the side of the original factory hole or elongate it to get the new strongback in.
Fiddly but well worth it for a good repair. Fill with expanded foam (just like factory did) afterwards and job done.
PS. Phil, have you seen the guy restoring a Lotus Carlton who's found a really bad engne repair with a welded together timing chain and split headgasket, on another forum?