Well it depends on what you want to do and how you want to do it. Are you good at cutting soft metals and/or do you have the tools.
I did a G5 case and put an aftermarket motherboard tray in it as well as mod out the G5 PSU case with an ATX PSU insert.
So you will have to look at how you want your motherboard supported insde the Pro case, mobo tray with or with out a rear IO panel. That depends on if your going to cut the back of your case out or not.
Front IO cabling for the power button, USB, Firewire and audio hook up. SO you will have to go from Apple to ATX.
Also PSU mounting at the top rear of the case where the original Pros PSU is and are you going to utilize the power plug opening or make a jumper cable.
YOu have some many choices and different approaches that it's all in your taste in how you want to proceed. Have a look at all the mods here and on other sites. Get some good ideas and parts you will need and feel free to ask the builders of these awesome mods exactly why they did what they did and what they used to do it.