Yep, the indicators just need to be visible from directly behind to comply with the vehicle lighting reg's for this age of car. I'll be taking a print out of the reg's with me to the MOT testing station come the day, just in case
There's just one more thing though Paul (in a Columbo stylee) - it's probably best not to assume that the rear wing seams are equidistant from the centreline of the car. I wouldn't personally use them as a datum, I would find the true centreline and work my way outwards from there. Then do the final fitting up by eye, and if it looks right, go for it.
Good luck getting the look you want and I hope you have a great start to 2013.