Drains in Wing Tanks

Wag-Aero has a special this month on quick drains:
http://www.wicksaircraft.com/catalog...879/index.html

I'm building a mongrel dog of a low wing airplane that uses wing tanks,
pretty much like the Hummel Ultracruiser Plus. The tanks themselves
are .032 aluminum. The question of the day is how to mount the drains
to the tanks.

2 styles and several sizes of drains are offered. The use either
tapered pipe thread or straight threads.

Obviously, a doubler will be needed to provide enough meat to fasten
the drains to. The question is to whether to make a fairly thick
doubler, and tap it for the tapererd pipe thread, or to use a somewhat
thinner doubler, tap it for straight threads, and put a shear nut on
the inside of the tank, before pro-sealing it up.

How thick should a piece of 6061-t6 be in order to securely hold 1/8"
NPT threads? I think I'm leaning toward that solution, with the
doubler OUTSIDE the tank skin to keep the un-usable fuel down a bit.
Looking at my "Machinery's Handbook", I'd need 0.2639" for full thread
engagement. Can I live with less than that? What do RV's use in this
area?

OR, am I barking up the wrong tree entirely, and I should grab a
welding flange, drill some holes in it, and slap it on with rivets and
pro-seal, and let it hang down.
http://www.aircraftspruce.com/catalo...alumflange.php