Have decided to install the Extended Range Tanks Fuel Bungs to facilitate extended range tanks if I ever decide to install them. I ordered the pair of flanges from SafeAir1 which were promptly delivered or a mere $59.50, undrilled with brass plugs. It turns out the Van's provided VA-112 Drain Flanges are almost identical, are predrilled, and have the same NPT fitting tapped. So I would go there first if you wish to do the same.
I measured and drilled the flanges on my drill press first with a 1/16 in bit, then with a 1/8 in bit. Will drill the tank ends and match drill with a #40 bit. and used AD470 3 rivets. Should be able to squeeze them.
