I don't know if you have come across
this thread where Clyde Lennon discussed what he was putting in his 406 (a Volvo engine).
I know Clyde spent a great deal of time researching the most appropriate modern engine and transmission to replace the original Bristol powertrain.
He took many things into consideration, including how much the car needed to be modified, engine weight, performance characteristics, acquisition cost, reliability and maintenance costs.
Weight is an important factor, because if the replacement engine weight varies dramatically from the original then you have to start modifying with the front suspension.
I can't remember how far the guys at Alpine Eagle got with Clyde's 406 project, but they may be able to tell you how much the car needed to be modified to accomodate the Volvo powertrain. I have some photos of the car somewhere, I will see if I can find them.