The article you are looking for is in the May 2007 issue of Healey Marque. You can view it online at the Austin Healey Club of America web site under Magazine.
The article is very complete but here are a few things I discovered when I converted my BT7 reflectors.
1. My original rubber pieces were cracked and the chrome rings were pitted so I purchased new reflector assemblies from Moss. I was planning on using the new units until I discovered I could not remove the foil from the back of the new lenses. Instead of having a thick piece of foil which can easily be removed, the new design has the foil reflector embedded in the back of the plastic lens. I couldnāt see any way to remove the reflective foil short of grinding it off with a Dremel tool wire brush. Since the new style lens didnāt have the Lucas name and my original lenses were still in good shape, I decided to reuse the originals.
2. The old lenses required a little modification to fit in the new rubbers. I had to remove the plastic ridge from the circumference of the lens. Itās only about 1/16th inch wide and was easy to remove with a Dremel tool. Once removed, the old lenses fit perfectly in the new rubbers.
3. Everyone who has done this conversion mentions the difficulty in cutting a hole in the rubber for the bulb. I finally nipped it out with a pair of diagonal cutters. Ragged, but it worked. Maybe the suggested freezing method would be better.
4. As I recall I spent about $35 for the parts and 2 hours of my time.
Given the original size of the brake lights, I think this simple upgrade is well worth while.