For most of these, Pinifarina made the rear mount and the factory modified for US front mount.
There is a trim ring - smaller in diameter than the inner curve of the fairing - that goes around the front mounted USA version so if you can find them you can use the original mount.
For the rear mount - Euro - the same headlight assembly fits. The rear cover is made in two pieces, with the inner piece a part that gives access to change the headlight bulbs - H3's IIRC (or H1's?). BTW - if you get the Carello (euro) or Cbie headlights, you can use H3 bulbs instead of the dorky US sealed beams.
To aim the lights from the rear mount, you have to modify the aiming screws so that you can fit a screwdriver into them from inside the wheel well; then you can aim them from the rear.
The inner wheel well cover should have a large round door with a screw or two that can be removed to access the headlights.
I've had trouble adjusting the lights without removing the wheel. Makes aiming them hard. What I do is set the car level 25 feet or so from a smooth white surface (garage door?). turn on lights and measure where they are pointed. Should be straight ahead with a sharp horizontal cutoff, with the beam rising to the right. Mark where this beam should be and measure the difference. Then remove the wheel and access the adjustment screws. Now turn on the lights again and mark where the beam is, and again where it should be, then adjust to correct.
Some of our suppliers have a stainless (or chrome?) ring that fits over the raw body edge around the light opening.