Find a common holley carb butterfly plate that is slightly larger than the stock 38mm, purchase six of them, and then bore you throttles to that dimension. You can get some sets that don't yet have mounting holes in them so you can have the new butterflys mounting holes made to match your thottle plate shafts.
You realize the bottom of the throttle body is taperd to a small diameter hole for the T, E, or S engine. If you open the bore up all the way down, you will have a shoulder where the body meets the head port and that ain't to good. Whether or not to open up you ports and the corresponding bottom throttle bore is for another thread though.