Not bad for your first prototype! Your next prototype will be much better. Here is what I think will help.
The heat from welding has annealed (weakened) the aluminum. With aluminum, this is a big change in the strength of the material.
If welding aluminum and not heat treating afterword, 5086 is nicer than 6061 because it has a much higher annealed strength.
6061-T6, 40,000 psi yield stress
6061-O annealed, 8,000 psi yield stress
5086-O annealed, 17,000 psi yield stress
From this you can see that:
* it is best to have aluminum heat treated after weld so it can be strengthened (added cost)
* or design around annealed aluminum
* or find another way to attached rather than welding
Also, I believe the aluminum from HomeDepot is 6063-T5 (21,000 psi yield stress) that is annealed to 7,000 psi yield stress at the heat affected zone of the weld.
is a nice reference for material strength, but your material supplier should be the best reference.
I think if you found a way to bolt the fuselage on, it would work as you intended. Also, if you want to continue using the square tube in stead of a square solid, it might be worth getting 6061-T6.
I hope this helps.