customizing spring steel bumpers

i recently tried narrowing a 50 chevy bumper and found out the hard way that they are made of spring steel, i tacked it up (mig) to test fit and all of the welds broke as soon as i put any pressure on it, i then tried annealing it but that didnt help at all. Now i need to narrow a 48 plymouth bumper to fit a 41 ford but im pretty sure its also made of spring steel. so my question is, does anybody know if 46-48 plymouth bumpers are really spring steel, and is there any different type of welding that would be strong enough.

 

Virtually every car before 1949 had spring steel bumpers.