Hi. Im sorry because i cannot really answer your question, but had a thought for you.

You say you can buy the 70w fittings with no hassle?
Trying to build a ballast and light fitting of 150w is hassle and costs more?

As you want to ADD these to existing lighting, i would buy the 70w lights and spend the money saved on building the 150w light on a fan to blow across the underside.

When i put my hand under my light (600w hps) all i can feel is the cool breeze from the fan.

Here is a pic to show you what i mean. Although my light is air cooled the extra fan means i can get the light much closer to the tips.