Hmm...
The drain plug seems like a reasonable place to go.
You seem like you have thought this through pretty well, but I just want to make sure I point something out... If the vent on the external tank is lower then the fuel level in your primary tank you are going to have problems.
How I would personally go about it would be to drill and tap a couple of barbed fittings onto the existing tank. One near the top and one near the bottom. Run hoses to the new tank, top and bottom. Make sure the lower fuel level of the new tank is above the lower level of the existing, and the upper level of the new is below the upper level of the existing.
I hope this would go with without saying... but drilling and tapping on an fuel tank isn't the brightest idea, but it could be done safely.