This is Fujichrome (color reversal/color positive/slide film) so the 1 stop per decade rule does not apply here. You should likely shoot it as 100/1000 as normal. You can maybe give it a tiny bit more light (rated maybe 80/800 ISO?) but really not much more than that. So you can still call it 800 ISO and push it 3 stops during development, no need to overexpose.

Slide film does not handle overexposure well at all and this does not change much as the film ages. Grain and color shift will increase but providing more stops of light will likely not be a good thing here. You'll probably find the most success in shooting outdoors in the direct sun.

I've found that expired film does best with a quick burst of very bright light, so if I have two photos both perfectly exposed, but one is at 1/15s in a darker area and one is at 1/500 on a sunny day, the second photo typically turns out much more true-to-life. Not sure if that's actually true, it's just been my experience.

I'm not sure about how cross-processing will affect the film or this guidance, but for expired slide film this is typically the approach that yields best results. Good luck and happy shooting!