Modern structural adhesives can be excellent if applied properly. They are even used for aircraft structure.
You would need to cut the apertures in the rear panel to exactly match the Mk.1 adaptor plates, ensuring that the alignment is spot-on.
Then make up doubler plates to provide flat lips about 10 mm wide which you could bond to the rear panel. In fact you could also use small countersunk screws to ensure the doublers sit flush to the curved rear panel.
Once the doublers are fitted and the adhesive is fully cured, the Mk
1 adapter panels could be bonded in place, again with smallcountersunk head screws to ensure full surface contact.
That would work well so long as the application of the adhesive is done absolutely correctly and all old paint and primer is first removed and all surfaces de-greases.
Finally a light skim of body filler would complete the job.
Edit. : Just to add that when I recently welded a pair of MK.1 rear light plates onto a car I did get a bit of localised distortion. It was easy to remove with a hammer and dolly, but it then needed some body filler to get it right. Using an adhesive should work very well, but the key is to do it exactly as specified by the adhesive manufacturer.
I think if I do another I will bond it in place with a Henkel structural 2-pack adhesive. I used some 'Tiger Seal' as an adhesive on my Land-rover Discovery 300TDi recently in a non-structural area and it has worked fine. With a better proper structural adhesive it would be even stronger.
Edited by Cooperman, 14 January 2021 - 05:35 PM.