It's not the vibration that I was talking about, an oxy/acetyl torch is mother-effing hot, thin metal is going to have some issues with a torch that hot. When I think about it it does occur to me that I don't think I've seen welded fenders, think they were all rivets or bolts.
Brazing is by far a better idea, but still kind of iffy, I've tried brazing thicker pieces of steel and still managed to burn through. If anything soldering is the best option just because you don't have to reach the melting point of steel for it to work, you only have to get the metal to the temperature the solder will melt. Home Depot has silver bearing solder meant for metalworking, it will take to most forms of steel, and of course will work on anything silver likes to meld to, so you can solder brass machine screws directly to the surface instead of a weaker hole with a bolt in it.
Your oxy torch will me more than enough for the (relatively) low working temp of about 600°F of a silver bearing solder, if you get water soluble Flux (for potable water systems) then after joining you can simply wash it off with a wet sponge, no hard Flux to scrape, chip, or sand off.
Sorry for the long read, I just don't think welds or brazing is the best way to go at it, both have to be done after forming, you can heat parts from the back and wet the steel parts with a layer of solder before forming the metal, then form it, and then quickly join 2 parts with some fast heat and an extra lick of solder with little chance that the heat will warp it out of shape. The best part is he can practice using the torch without jumping right in to melting steel, this gives him a chance to learn to just keep an area a steady even working temperature, probably the hardest part/learning curve of using any torch anyway.
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