A833 shifter plate removal.

cudascott said

The degree on the heads and the plate make for a high resistance and it’s tough to break them loose with no corrosion involved. I find it easier to use a 5/16 drill bit and drill the center and the head will break off. After that the remainder of the bolt will almost screw out by hand it’s the surface area of the head not the threads that’s making them difficult.
If you can’t get a right angle drill in there your gonna need a good impact wrench and Allen socket to break them loose.

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I have to agree with cudascott. Corrosion occurs around where the back side of the bolt meets with the aluminum plate. Two dissimilar metal's, especially when steel & aluminum meet, will form what is call galvanic corrosion, & make the two metals kind of weld each other together. Drilling out the screw/bolt heads may best solution. Otherwise soak them GOOD with PB'laster (I swear by that stuff too) lower the trans with a jack as much as you can, (remove the trans cross-member etc.) & use an impact DRIVER, not a gun/wrench, with a good fitting Allen bit & try to remove them that way. But I think the idea of drilling the heads of the fasteners off is your best bet. Some heat wouldn't hurt either, if you can do it without burning up anything. Good luck, & let us know how you make out.