Hi Kruger, I based my explanation on your ISO 100 film, if you exposed it at 1000 ISO, then that is underexposed, you work out the number of stops by doubling the ISO number, i.e.:
100 > 200 > 400 > 800
1000 is close enough to 800, so that means it's 3 stops underexposed. I would probably go for a 3 stop push, but you may want to talk to your lab guy.
A two stop push would leave your shots underexposed by 1 stop, which some people do on purpose to get more saturation on slide film.
Talk to the lab, see what they think.