The only thing I'd worry about is you have a lot of leverage on that brake booster with that long rod. I've dealt with hydroboost a lot and know that it puts out tons of braking with a light foot. But if you decide to slam the brakes hard your feet put out a lot of pressure also.
In the pic I have circled the joint end in green. That area can move in any which way it needs to. Now my thought is with hard braking this area will kick upwards hence snapping the threads circled in red.
From my experience it is best to cut the booster shaft (at red lines) and machine the area between the red lines down to 1/2" so it slips inside your tubing like a bung. Weld it there, then grind it smooth and sleeve over the whole area and weld the sleeve, no chance of breaking then.
Like this:
I know, very crude drawing..... Good thing I'm no photo editor.