I'm not sure about the studs. Try looking in the back - maybe use an inspection mirror - and see if there are threads there that the studs are mounted in. In short, try to figure out how the studs are anchored.Its not long Enough to come thru the throttle body at all.. Do these studs come out?
The side that isn't broken is all jacked up too. I guess the washer and bolt when assembled (not sure what it's called) won't stay in place.
As suggested, you might try unscrewing them with vise-grips. If you break the studs off, you can try drilling them out. If you try that, stuff some rags in the throttle body opening so that shavings do not get down in there.
That said, if you break the studs off, maybe the next step is to go to the junk yard and look for another plenum.
In brief, try to get a close look at the studs - front and back - to see how they're attached and see if you can get them out.