Hello mate,
Like you i'm restoring a White 3 door Cosworth, although it's White with Red and Blue stripes at the moment as well as having some very fethching 'Chic' graphics too (don't ask) !!!
The boot decals on a White 3 door are a dark grey/very nearly black colour (i bought a repro decal from PJG Graphics for £7.50 and it's spot on) and will look the part when it's finally painted.
The aren't any standard pinstripes or any other decals on a standard car so you've owt else to worry about. There are a fair few decals under the bonnet (1xairbox, 1 x slam panel, etc, etc) and these are often availble as repro's at various Ford type shows for between £15-£25 a set.
I wish you all the best in trying to get yours sorted and beween us we can put another two Diamond White 3 doors back on the roads.
If i can help you any further than just let me know.
CheeRS
James