My own solution for mirrors was to install round spot units that clamped to the leading edge of the door window frame (a real 1950's solution) on the '65 544 I drove for a couple of years.
The first time I ever drove in a track event (with my '69 144S in 1997) I learned something that has always stuck. The fellow (an experienced racer) conducting the drivers meeting spent a fair amount of time talking about mirrors. People are naturally prone to adjust mirrors so that the flank of the car is visible - confidence arising from the established point of reference. This is what makes for "blind spots". Both side mirrors should be adjusted outward enough so that you must lean over a bit to see the flanks. What happens is -- as a vehicle approaches from behind and passes on the right or left it leaves view from the interior rear view mirror - but with the sideview mirrors adjusted outward the passing vehicle will appear in view just as it leaves the interior mirror. It works. -- Dave