Install of bulkhead fitting is simple, first, the squiggle washer likely goes in trash, just a sealing washer goes on the flange and hold/tighten securing nut on exterior, to get it in the tank is easy, feed a string from sender hole out the bottom hole and slide the fitting down the string, all else held/ tightened from exterior…..says the boiler guy who worked on tanks….a lot
If soldering pretinning is critical on sheet metal (or you are setting up to fail), thin puddling the solder is 2nd, capillary attraction alone is great for copper fittings where bonding characteristics are ideal and gaps are relatively uniform, and designed for it, concept good but much much harder to achieve in reality on a flat thin sheet metal part. Getting a bond layer to the tin is not a given, the brass will be very forgiving, galv tin ….not so much….
Brazing also an option, but not recommend for this application unless experienced ….the heat can easily create issues on the sheet metal.
