Are you going to Heat-treat that swingarm, after you welded it?
I do not think it needs heat treatment of any kind just for that minor modification.
When people say words like heat treat, I do not think they know if that means stress relieving , completely starting over , anodizing ,etc,. And where would they even get that kind of expertise and service facility?
I worked in manufacturing and I welded aluminum weldments and all they ever got was anodizing, The parts lasted 30 years out in the ocean.
The only time an aluminum used seaplane aircraft part was stress relieved, it cracked from trapped corrosion pockets when heated up.