Stick with the lucas electronic dizzy with a working vac advance and timed up properly. quality igntion parts from simonbbc and a bosch coil.
The cam, followers and chain are more likely to be the problem than a dizzy unless already changed.
The 500 weber is really too big for a 3.5 v8, getting it to run correctly is tricky. My own 3.5 is turning 200bhp still on strombergs!
Mallorys are pointless (geddit!) without electronic ignition, you are quite right it is a step backwards