Based on my experience, you're NOT going to get the screws out intact. Mine were rusted where the screws extended thru the shell of the tank, so short of cutting them off inside, there was no way they were going to thread out, no matter how much kroil or whatever I used.
I thought one of mine was loosening, but it was in the process of twisting off. After that, I just snapped them all off and drilled them out. Started with a 1/16 inch bit, then working up till the shell could be picked out. Ran a tap (same size 6-32) thru the holes and they're all good to go.
