Is there any gap in the radio halves?
When the screw head is stripped, meaning the phillips head (cross) is gone, I have had to drill out the head. Go slow, take your time. Once the body was off, then use Vice Grips to get the screw out.
When the post is busted off and spinning with the screw, I was lucky enough to be able to pull the halves apart enough to get needle nose plyers in there to hold the post and keep it from turning to back the screw out.
When the screw is stripped in the post you have a few options...
1) Remove all other screws, if possible use a screw gun / drill with screwdriver bit, pry the body apart, putting pressure on the screw and turn the screw at the same time. You might need help from another person for this. This can be tricky. If you have to, use a big screw driver to pry the case open and put pressure, but be careful not to tear up the case or anything inside.
2) If you have enough of the screw backed out and can pull the case open enough, using a pair of dikes (plier sized cutters) reach inside and cut the screw. Position the dikes so that the FLAT side of the cutter face is facing the screw head end of the screw and if possible, cut leaving some screw shaft left to grab onto with vice grips to remove it once apart.
3) This is the last case scenario and something I absolutely hate doing. Using the blade from a hacksaw removed from the handle, slide it in between the case halves, pull apart the case as far as possible and saw the screw. There are serious issues with this beyond damaging the case or the screw post and that is getting all those metal shavings INTO the electronics of the radio
This is BAD BAD BAD! Do your best to position the radio so the metal dust falls AWAY from the electronic components and afterwards blow out the whole inside including all pc boards with compressed air (while wearing safety glasses).
If possible, go to the hardware store and buy new replacement screws for all of them, you might need slightly bigger screws for the enlarged holes, test them in the posts before putting the back on and don't use too big of screws that will blow out the posts.
And yes, I have done everything described and learned through horrible trial and error
Best of luck!