Nice start, but I do not think you will be able to use a full sized drive in the machine. If you want to go with a drive I'd go with a laptop drive and mount it with a bracket of some type. There are other builds out there that have done that. On my Sawtooth Build I had to cut the tray where the original drive carrier sat to make room for the power to the Logic Board. Which you will have to do too because I can see that it is touching the plug. Also find a 120mm fan to go in the same place as the original. Temps in the G4 cases without that fan run into the danger zone. have it so it exhausts the air out like in the original config as well. Another reason for going without a DVD Drive is you can out an 60-80mm fan on that shelf to blow air over the board and add circulation to the inside of the case.
Definitely check out other G4 builds on here to get an idea for cooling before firing it up. The last thing you want to do is kill something you put a ton of work into.