Hi Ray,
Actually pretty easy thing to do if you have the proper solderring iron/gun.. I use a large Weller soldering gun but a 60 watt or higher iron will work fine. Use electronic solder 60/40 available from radio Shack, you typically won't need any additional flux.. Do not use acid core solder or Stay Bright flux, not necessary and just causes corrosion problems unless you know what you are doing and how to clean it well.
Tin both the tank and the brass mount, touch the iron to and apply a thin coat of solder to the parts you will be joining. I place the tank in a small vise with the end up, set the brass mount on the tank and apply the iron to the topside of the brass mount.. Apply a bit of solder to help the iron heat up the brass, than apply a bit of solder to the edges between the mount and tank. The solder will wick in between the mount and tank.
You may move the mount out of place but just scoot it back with the iron or figure out some way to clamp/hold it in place while you solder it.
The iron won't heat up the existing tank joints enough to melt the solder unless you get carried away and hold the iron on too long. The parts you are soldering need to be hot enough to melt the solder or you won't get a good joint. If you just touch the solder to the iron to get a small bridge between the iron and item, this helps the iron transfer the heat. Than move the solder off to the side on the item you are soldering, you will know you have it hot enough when the solder melts.
Lots easier to do than explain...