Bottom brackets (39x68mm) are available with a seat hole (28.6 or 31.8mm).
I have used these brackets for (lightly loaded) children's bicycles. I used a T47 bracket for my own bike with the down tube in the bracket. Always with fillet brazing.
The new frame is for my son who has more weight (80kg) and power.
The thickness of T47 is 2.5 mm outside(2x 12.5 mm) and 1.6 mm thick in the middle (43mm).
For me it is easier to drill a hole than to place the miters exactly perpendicular and to keep the exact length.
It is also the case that the rear brake cable runs through the downtube, bottom bracket and left chainstay.
Question is : how should I imagine shell distortion : less stiff, deformation, breakage?