By: Gordon Watkins, from information he compiled from the 356 Talk List
Striker plates and the backing plates are often rusted in place and it's difficult to remove them. Here are some tips from members of the 356 Talk List:
I recently had the same problem. Got 2 out of 3 screws out on each side but the last 2 were tough. I used a screwdriver bit in a variable speed drill, placed it into the screw head, leaned REALLY hard against the drill and slowly started it up. Of course the key is not to ruin the screw head. For awhile I was worried I was going to burn up the drill. Eventually I got them out.
Have you tried one of those impact wrenches that you hit w/ a hammer. Be sure to use a large Phillips bit. If you aren't familiar w/ them , they are cylindrical approx 1 1/2 " dia x approx 6" long. One end has a chuck to accept various bits...slotted screw, Phillips screws, and a socket adapter. It is spring loaded on the inside and can be set for tightening or loosening when you strike it w/ a hammer. Avail at Sears and most FLAPS.
Use a air-cut off tool to melt,vut the plastic/rubber, and the the heads of the screws off...the carnage falls out in the back, and a new striker and bolts you were going to install anyways, so no great loss.
Probably not your first choice, but if they are really hosed badly, you can cut them out and fabricate a section to weld in...see my web site to see how GearHeadDave did this with my vehicle. http://www.embarassingmoments.com/356/toms%20356.htm
Another last thought, is there enough material there to weld on a bolt to the end of the screw head ?....then, turn the bolt out and the screw will come too. Actually, I don't think it is a screw, it is just a good size bolt with a phillips head on it....and Brad Ripley has nice replacement kits (I know I sound like a broken record)....see my web site for pictures of the NLA replacement kits.
You would almost never have to cut it out. The drill out (easy on phillips screws, harder on slotted) is first choice, second choice is grind off the nut part of the plate on the back side and then it will fall off easily. You should never have to even touch the body metal (though grinding heat might damage paint). Before doing this, look at parts blowup pictures, or check out the parts from a friend in your hands before you start.
Drill out what is left of the flat head bolts with e 3/8's drill and everything should fall out on the ground for you. Of course you will have to replace all the pieces with new!! You would have need to do this anyway
Drill the heads of the screws off the plate in the inside of the fenderwell will fall off. You can then heat the plate and remove the screws clean and paint and reinstall. With a little luck you can reuse the latch.
Drill out the screw heads. Then you can remove the remaining threads with the female part in a vise.
Regarding your striker plates... what about just drilling a hole into the screws and then using an "easy out" to screw them out.
Go to your friendly neighborhood Sears and buy a manual impact driver. It looks like a double-jumbo screwdriver with replaceable "bits", but it works by hitting it on the back end with a hammer. The internals translate the impact to rotation. The benefit is that the impact also drives the screw bit into the screw at the same time. It's really a super-handy tool to have......
The problem with this method is that you can easily bend the metal of the lock post. Drill off the heads, the nut plate will fall off. You can put a little heat on it and grab the remaining part of the screw and unscrew it.
Start with a small bit. 1/8 and drill a pilot hole then enlarge the hole in steps and the drill will not slip and scratch the receiver. This is the only way to do it without damaging the car. In my opinion.
You could drill them out. I've done this successfully on door hinge screws before. Start off with a smaller drill bit and gradually increase as needed.
I have had some luck with the screw extractors Sears sells. You may have seen Bob Villa selling them. You have to put a lot of pressure on them and they will destroy the head of the screw, so if it dosen't work then you have to drill them out.