Some have found that they can adapt C@pri headlight backing plates to suit and use outer lights with the sidelight included. AFIK it is a reversible conversion. However, I personally think a nice unmolested Viva with original rectangular headlights is a nice car. Fit an SL grille or paint the original delux one black if you want a change but keep it original.
Fitting twin round headlights really is very much the thing to do, but if the original rectangular headlights were good enough for a GT40 then they are good enough for a Viva.
As far as painting the original one black is concerned I would try to make it look a bit like the HB GT grille and retain the bright metal border and paint the complete centre section black including the middle upright 'V' bit. A subtle little bright badge on the lower passenger side would complete the look.
Probably the Delux grille will lend itself better to painting than the SL grille in the picture below as you could get a thinner silver border.
Editor of the VVOC Magazine ReVival
"When you have a Firenza like mine, who needs any Minis?" copyright David Maxwell 1979.