Gap for brazing thin casting? Non-automotive

Took a while to get all the patches in the wall and pieces test fitted.
1751579740254.png

1751579877786.png


When the paint is dry I'll put the working register in. and then the grill. But this is a test fit with a spare grill.
1751580400146.png
Still working on a copying the control rod threads for the register that allows heat to go up the flew. Need it for a similar second floor control.
1751580443800.png

Looks like 24 threads per inch but its a rounded thread form.