I know you've said you don't want alloys but this idea might just work - you remember the Minilites that Volvo had made for the P1800S? In particular a white "as seen on tv" P1800S? I'm referring of course to the one used (and later i think owned) by Roger Moore in "The Saint".
That made Minilites on the P1800 acceptable as a period accessory and i think Volvo used them on their rally versions of the P1800 and also the later 142. I could be wrong on this but if not, it might be another option. Being "period correct" wouldn't alter the look too far from standard as well.
I'll have a look for your thread on the 140 forum!