Would there be any significant differences between going with a stock TB bored out versus the larger MB TB???

I don't believe there would be any performance difference (or at least noticeable) as long as they are the same bore diameter. However, you raise an interesting point about the structural integrity of the part - if this was suspension or a moving item I would be concerned, but seeing as though it doesn't really move or have many forces applied to it I wouldn't be concerned. Seeing as though both throttle bodies (68 + 74mm) will fit onto the same air intake tubes (which is before the butterfly - so the initial intake is the same size) there may just be far more material on the stock 68mm which can be machined away to make it a 74mm unit. I don't have a throttle body at hand to see how they are built up/assembled so I'm assuming the above.

If you look at the pic below posted by psuperit, the butterfly is significantly less than the inside diameter of where the air-intake assembly attaches to - reitterating my assumption above.