A bit old topic but I have just these plans with my Bagheera.
Replacing Simca engine to a VR6. Have done some measurements and with tank located somewhere else (front) so could VR6 engine fit in the back without much changes in the existing chassis.
Problems I have to solve is weight. A VR6 (ABV/190hp) motor with gear box is at about 180 kg (anyone know what a Simca 1.4l motor with gear box weighs?), with this weight back there the chassis must be strengthened, and new mounts placed. There is also issue with the cars total middle weight pint. So if weight point doesn't change so much I will start with modifications this fall.
One of the main reason why I want a VR6 engine is engines deliver so raw sound with open sports air filter and other cams.
It is mentioned in the thread that it is a bit of dreaming but without dreams reality becomes boring

Cheers!