Need to install new power supply on my Dimension 8400

I need to install a new psu on my dimension 8400.  I have the new psu but need to cut the back panel because the power plug the the old hole for the power plug do not match up and because my new psu has a power switch which the old one did not.  My computer has the original psu in it which was a 350 now I have an Antec Earthwatts 650 watt psu (EA-650) which Dell suport said was compatible.  They did not tell me that I would have to cut the back panel.  How do I cut the back panel so that I can install the new psu?

Kim