If they are wired up when you measure them, it doesn't matter which battery you measure across, you are measuring them combined.
On the LED's, I bought these: These eBay bulbs
but it doesn't look like they are available anymore.
When I finish my LED writeup, I'll recommend going out on eBay and buying a few different types to test out - before you order a whole bunch of them. At $5 each, it's worth the investment IMHO.