Rick, assuming the thumb safety is already installed, the pressure on the slide stop's detent plunger won't be any different, whether the slide is assembled or not.
It sounds like cciman is having trouble pushing straight in with the slide stop. What I'd try is pushing it straight in before it's lined up in the disassembly notch. Then you can try pulling back the slide to its proper notch and pushing in the slide stop the rest of the way. I've used this method on more stubborn detent plunger springs and it's worked well.