The stock tail lights came with just red/clear lenses. The Euro tail lights had an amber lens for the turn signal section. The housings for these two styles are EXACTLY the same. Remove the screws, remove the lens - reverse the procedure to install Euro red/amber/clear lenses.
The Group 4 housings do have the exact same footprint as the stock tail light housings. The lens to housing gasket is the same. The electrical connector plug is an exact plug-n-play swap. The Group 4 housings are not as deep as the stock housings and the mounting studs will not protrude into the engine bay area to accept the nuts and washers used on the stockers. You can easily get around this problem by using 6mm barrel nuts (couplers) but you will probably have to search for them, and the little buggers are not cheap.
But while the Group 4 lenses have the exact same footprint as the stock lenses, and will 'fit' onto stock housings, the housing is unique to the Group 4 style.
The link posted above has a photo of what was done to 'adapt' a stock housing to Group 4 lenses. Quite the creative, Rube Goldberg conversion. And while I admit you would get light shining through the Group 4 lenses with this conversion, I can't help but feel the "look" of the lighting would not be right. Bulbs and the round reflectors would not be centered in the corresponding lens sections. Part of the Group 4 'look' is the circular light source coming through the lenses, and a conversion won't provide this. True, only noticed at night, and you won't notice it at all while you are in the driver's seat.
So, during the day, you got 'the look' and those following you at night probably wouldn't notice anything amiss.Some of the vendors sell complete NOS sets, and P.I. lists NOS lenses and a conversion kit.
I know of three Group 4 lens styles. Carello and Altissimo are both original lenses. Altissimo lenses are about 1/4" deeper than the Carello lenses, meaning they stand out from the car a little farther; and the lens looks a bit different for the reverse lens. Third style is a reproduction of the Carello style by an unknown company. You can however identify them as the red reflector is embossed with the word "superreflex". These reproductions were cast entirely from clear plastic, and spray on dye was used to create the red and amber sections. This coating is prone to flaking, and may over time degrade and fade. One eBay seller offers "NEW" Group 4 lenses, but despite repeated questions to him, he refuses to answer whether these are Carello, Altissimo or the cheap "NEW" reproductions. That makes me think they are the reproductions. I have a set sitting in my garage that I really would be embarrassed to sell. They look okay, and most folks would never pick up on the reproduction aspect, but I don't like them.
I currently am listing a restored housings/NOS lenses set on eBay. Buy it now is $335. This is an Altissimo set, not the Carello style which I believe was the style always used by De Tomaso. Most people do not even notice the difference between the two unless I point it out to them. To see the difference in the two lenses, view my auction where I have photos of Carello and Altissimo lenses.
View the auction, and photos of housings/lenses at -
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewIte..._W0QQfromZR40QQfviZ1
If I didn't answer everyone's questions with all this, just ask.
Larry