Another solution would be to make the hole all the way through to the inside of the manifold, since you have deleted the heater plate, your stud is long enough, so put a self locking nut( I don´t know how you guys call this nut on the states...)on the inside of the manifold, put some double compund(stuff that suplements aluminum, here in Spain the name is Araldite, or Nural 27) so the stud seals and sticks to the manifold,let it dry for 2-3 hours, and then just like normal, put your throttle body, nuts, and you´re done!!
You really wouldn´t need the nut on the inside using the stuff I´m talikng about, it´s just for safety.
Just my idea...