My money is on an earthing problem. Starters take a huge current and what is perfectly good enough for lights etc, is not good enough for a starter.
Remove both ends of the earth strap, clean up, apply new vaseline and bolt up firmly and try again.
Won't help either but the Lucas starter as fitted in that period is definitely of the grunt and heave school wheras a geared starter will whiz the engine round. They tend to be expensive, but if you can get one for a sensible price, worth having, even with a standard crossflow.
Dont forget to check the engine earth lead as its insulated by its rubber mounts. This can either be on the engine or gearbox and will connected to the chassis.
