Danny,
Good work getting your SA brake arm correctly attached.
Tank building... I build mine in aluminium and use Alu-build brazing rods to attach the alloy filler neck. a 40mm hole saw cuts a perfect-fit hole to make the filler neck a tight, snug fit with no potential for movement whilst it's being brazed in place.
For a steel tank, I'd use 'Easy-Flo' silver solder to attach a filler neck to a steel tank. When the 650 degree melting point of silver solder is reached - workpiece should glow dull red - it very tidily flows along the pre-fluxed joint like water. Just keep pointing the flame to where you want the solder to flow. Biggest benefit of silver solder? Almost zero clean-up of the joint is necessary other than to wash off flux residue.
I imagine you've already had your fill of hand-filing brazed joints on your bike frame. How are the callouses? Haha.