They are just too heavy for the front of the car making them handle bad and the chassis flex.
They are a tight fit in the engine bay of any e-body and get hot. You really want a hot oil fume smelling engine making your eyes water and your clothes smell while trying to look cool then fit a oil buring hemi in there for sure. You ever tried changing the plugs on a 440? Why have the hassle if you don't need it for performance?
To me a convertible is not about all out straight line performance. It is about cruising around looking good and having fun. A 340 or 360 can be stroked to well over 400 inches and make 500hp if you really wanted performance but I think a nice mild small block is an ideal engine for this sort of car.
Just my opinion of course.