Hi Darren,
I just did this to the RH fender you sold me. One thing that I didn't take enough time to think out was that the hole for the 73-74 is quite a bit larger than the 70-72 signal lights. If you enlarge the hole equally on all sides this moves the light cover over to the inside edge by approximentaly 1/8" of an inch. This does not sound like a lot, but the light is already close and you will notice it. Correctly done, the edge of the hole is 3/8" of an inch from the inside edge of the fender where the hood rests. The hole size you need is 9/16" for 73-74, and I think 3/8" for 70-72. If you enlarge the hole you currently have from 3/8" to 9/16" even all the way around, the centre of the hole shrinks the distance to the inside edge by half the distance of the enlarged size of the hole. I hope this makes sense. (3/8" - 9/16" = 3/16" divide by 2 = 1.5/16").
Bottom line... increase the hole size on the outside edge only, not the inside edge or make sure you do not decrease the distance of the edge of the hole by any less than 3/8".
The picture below is the correct measurements for the 73-74 e-body signal lights. The large hole is 9/16".