Back in the day when I had to run around in the dark with a gun and a map we used to buy cheap torches and 'pen in' the lens with permenent red pen, I agree that it would be prudent to do this to the bulb in the ammeter. Also with regard to placements of the bulb, I havent taken the thing apart yet to paint the interior but the bulb would be either on the lower back or possibly mounted on the side at the top...time will tell. The plastic is of good quality and I dont see any problems with painting the interior or drilling for the bulb holder, will just need to wash the plastic in soapy water, lightly sand, wash again, dry and paint.
Residue of mould release agent for plastic components can linger for many years. I'm in the process of building a 1967 Tamiya kit of a HONDA RA273 and thought that the residue would be gone after 40 years..... the rest of this sentance writes itself.
I did ask on this forum about the colour of the internal dials and nobody could help so I'm going for ferrari red.... so every time I look at the gauges it will remind me why the GT40 came into existance.
The photo of the original does have an extension on the zeroing screw, I will probably keep mine as is but check its zeroed before I refit the glass after painting.
Andy