Hi gang, first post. I'm a semi-retired auto mechanic with about 35 years in the "automotive heating and cooling" field. In short a radiator shop worker and then owner for 18 of those 35 years. I have worked with solder for a long time and have experienced what it is and is not suited for. If I may offer these few words of wisdom:
Solder is a like a plating process. Unlike welding, which liquifies both pieces of metal and fills with more molten metal, solder just "sticks" to the surface. So, if it ain't clean it ain't gonna stick! I used a product from the Barbee company in Louisville, KY called Tin Tastic. Neet stuff. It's a mixture of zink chloride, ground up lead, tin and zink in a paste form. Brush it on, heat it up and presto the surface is plated (tinned) and ready to take solder. I also used a nifty torch made by National. It burned natural gas or propane and compressed air right out of the shop compressor. Extremely adjustable from a weeny little 1/2" flame right up to a 6" long bright blue scorcher. Over the years I tried different blends of solder. Wound up using 40/60 for everything including fuel tank repairs. Never seemed to have a problem. I couldn't get enought heat with a soldering iron to do much good besides, the torch was much faster.
Guy's idea of a J type channel is a good one as tight joints are a must with soldering and the added mass of additional layers of metal will help to decrease warping on long pieces. If you can't make a J channel then try to do what Andrew Albrecht did. Notice in his picture of the end, he made a pan and then slodered (I think) about an inch wide seam together for a very strong connection between the two. Can't tell how he made the front seam but it's nice and wide, also for good strength. This guy knows what he's doing.
I don't know about brazing cuz every time I tried to braze two pieces of sheet metal together it came out lookig like a piece of corrigated roofing! Too much heat required me thinks. It's stronger than solder though.
How are you going to mount this tank? On a separate stand not connected to the motors vibrations is best. If you do mount it to the motor I would suggest using straps and cradles. EL brackets welded to the tank will eventually crack near the welds from the constant vibrations. If you've ever noticed a car or pickup fuel tank that's the way the majority of them are installed.
Anyway, if you you live in Canada there's a company called Spectra Premium in Windsor that sells all kinds of radiator parts, euipment
and supplies. That is if you find the need. If they can't / won't help you PM me and we'll figure something out. If you do call them tell them you're Doug's Radiator Shop as they won't sell to the public.
Thanks for reading my blather,
Bob