Help with replacing door latch on 72 C20

[ChevyC201972]

I am replacing the driver side door latch on my 1972 C20 and could use some advice. I am having trouble with loosening the 3 bolts that hold the latch - I have used PB plaster and everything else I can think of and they will not break free- my next option is to drill them out but I am worried that may create a problem when putting in the new latch. Also the space is very tight and I am concerned about getting the door handle and lock assembly all hooked backed up correctly on the new latch. Any advice on the best way to approach this project is much appreciated.