For a 1987 car, like my 87 Regal T, there isn't too much required for the UK.
Emissions are just a smoke test.
You need amber indicators front and rear. You can, if you want, fit Euro spec 1982-83 style indicators for the rear, or you can break in to the wiring and fit amber indicators somewhere on the back of the car. You can, due to angles of visibility regulations, even stick them on the parcel shelf inside the car. With the new LED's you can have amber LEDs blow through the red lenses. I was going to have this done. Went to the trouble of sending a spare 86-87 tail lens I have to a guy to design something and he has disappeared, with my spare lens. Bugger.
Front amber indicators are okay, just use the stock ones.
You need to show a white front indicator (ie. marker) light to the front independant of the headlights, but again, that is okay as the US style rectangular headlights are available here with an integral 5w small bulb.
Luckily, the front side marker lamps on the 86-87 cover you for showing an amber side repeater for the indicators.
You also need to fit a fog lamp at the rear.
That is about it.
Here is a photo taken at the Mopar EuroNats in 98 showing some variations:-
Dave's car (the left hand 86 GN) has the 82-83 Euro tail-lights and an rear fog lamp under the bumper.
Ian's car (the right hand one) uses the reversing lamps, tinted yellow, as the rear indicators and has a rear fog lamp.
My car is in front of Ian's and I didn't bother with anything, just ran it on US rear lights (including flashing brake lights) but fitted white front sidelights.
Colin's GNX is hidden behind my car.
The other photo shows the view in the other direction.
Plus, one of my GN back in 96 or so at Avon Park Raceway.
Apart from fitting healights with a sidelight built in, I have not done much with my Regal T, but I do have a set of spare 82-83 Euro spec tail-lights if the authorities get picky. Quite lucky in that the Regals have such BIG double flashing brake lights and a nice steady HMSL.
All the best
Sandy