1. Drill the lower out to 31/64"
2. Go to the auto parts store and buy some 1/2" oil gallery plugs. Clevite part number is 219-3169.
3. Using a blunt punch, gently tap the plugs into the holes you just drilled.
4. Cover the plugs with JB Weld.
5. You're done. Slobber the sh*tbox together. Use the new GM aluminum gaskets, of course.
If you're a belt and
suspenders kinda guy, you can also plug the upper while you have it apart. The UIM passages as cast are .375. Use a 7/16" drill to go into the passages a short distance, then a 1/2" drill far enough to seat the plug. JB weld it. You're done.
(This UIM was also sleeved.)
As to icing, I think it'* extremely unlikely. The intake charge passes by that plastic ring far too quickly to be heated much, and if you look about 1" below the throttle body, you'll see a loop of the exhaust. That loop probably provides all the throttle body heat you're ever gonna need, unless you're motoring around north of the Arctic circle.