those are a bitch to get out.....and by the time you do, usually the threads in the head are fubar'd
fyi, on that front one nearest the thermostat, the hole in the head goes all the way through. go to the parts store (if the threads are stripped out in the head)
and pick up a longer bolt and a nut, and you can nut that intake bolt from the bottom of the head.
I'm not sure if that will work in the back of the head or not.....good chance it doesn't overhang the block.
if it doesn't, I wouldn't drill that bolt out, because the intake bolt hole goes down into the intake valley, and you'll get metal shavings in the intake valley which will destroy your motor.
If that's the case, pull the heads and fix/upgrade/replace as necessary.