Hmmm,I'm not so sure of that.
When you bend a stick, where does it break?
1) The addition and welds are in the heavily stressed middle of the arm.
2) Additionally you are welding the heavy gauge insert piece to the thin chromemoly.
3) With the shock mount where it is, you will have the exact same travel as a stock swingarm.
I've done several 2", 3" and 4" extensions by chopping and welding a piece in here:
It is 1/4" mild steel, to 1/4" mild steel. Easy to lay the 4 x 4 inches of weld into it with stick.
Welds great and so far no problems with breakage.
I also weld a horizontal 1/8" plate between the additions for rigidity. Barely visible here:
Since it is extended behind the shock mount you have longer travel and softer springing.
Even the 4" extension on my son's Blasters has stood up very well so far.
Always good to inspect them.
Steve