Welding on an aluminum frame or fork is probably a bad idea, as the welding heat softens the aluminum permanently (unless you have it heat-treated again, that is).
Welding on steel is possible, but bronze-brazing it would be safer.
Thin steel will tend to carburize a lot on the back side and flake off, weakening the metal around the weld. Brazing is done at a much lower temperature, and totally avoids this problem.
Originally Posted by Venice Motor Bikes
I would mount the caliper on the back of the fork leg so the stopping force pushes on the back of the fork instead of trying to tear it off the front of the fork!
This is not a good idea.
Normally the caliper on the front side gets pulled forward, and the wheel axle gets pushed UP into the dropouts.
If you mount the caliper on the rear of the fork blade, then the caliper gets pushed into the fork blade (no problem there) but the caliper's stopping power pulls DOWN on the axle. A few people who tried this have had their front wheels come out while riding because of this when they stopped hard. The couple times I saw pics, the dropout itself cracked in half.
Motorcycles can do this (mount the front calipers behind the fork blades) because they do not have slots in their fork ends--they have holes, and a thru-bolt.