I think you will be OK with the standard #8-40's even thought they are a little loose, I tried a few and tightened them as hard as I would in anything else without a problem.

Top are a couple of the ones from a Pachmayr kit, you can see the heads are slightly smaller than the hole -
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The front one here is from Brownell's, it's even smaller with a head diameter of 0.240", I think some of the newer ones you get have a totally flat top. The rear screw is a #5-44 for Visible Loaders just loose in the hole to show a screw with the correct head size, I believe it is one I got from Apertures & More, the head diameter is 0.251".

I have three 435's and two needed work, I got frustrated while removing and replacing the hammer springs and took the time to make a spring compressor from some scrap steel I had laying around. The plan was to keep it very simple but it got more complicated as I kept trying to get it to work. Hammer springs are very easy to remove now so I no longer dread working on these.
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Gene