Does the 100-6 have a parcel shelf like the 4 cylinder cars on the passenger side, I mounted my radio under the parcel shelf using existing holes in the shelf I attached mounting brakcets to. Then simply routed speaker wire under the carpet to the back where I put speakers behind the seats, the really had nowhere to go back there and weren't noticeable, so all an easy install and very reversible, didn't even drill a single hole.
Isolated a fender mirror and used it as the antenna.
As far as the ground conversion, is pretty simple, unless you have a polarity sensitive fuel pump. there are instructions all over the net and on this forum that should come up with a search. Pretty much the same process for all british cars of the era, all have the same lucas electrics.