UK regulations state that flashing rear red indicators (which aren't also visible from the front) are legal on cars first used before 1st September 1965, otherwise they must be amber.
http://www.legislation.gov.uk/uksi/1...chedule/7/made
As the original Ferrari wouldn't have had indicators at all, anything reasonably period looking attached anywhere you like the look of them won't look out of place, as it's what an owner would have done for road use back in the 60's or 70's anyway.
Also, have you considered white lenses with amber bulbs? Would be legal and less obvious that they're indicators until you use them.
As your donor is pretty modern, you'll also need to read through the lighting regulations to check if your lenses need to be 'E' marked.
Project's coming along very nicely, btw.