What are the original dimensions of your 4:3 ProRes film transfers?
Why are you exporting via QuickTime Conversion to H.264 in a non standard frame size?
A DVD Video disc is always standard definition -
720x480 for NTSC or 720x576 for PAL. Even when it is widescreen 16:9.
A DVD Video disc uses MPEG 2 video and a Dolby Digital AC3 audio file as its basic components.
You are going from ProRes to H.264 to MPEG 2 - three conversions.
It is not possible to make the MPEG2 and AC3 files via QuickTime Conversion - iDVD does that. Edit in ProRes - Export as ProRes using Current Settings and Make Movie Self Contained.
You will have far better results using Compressor to make your MPEG 2 and AC3 though. Bring those into DVD Studio Pro, where you also have much more control over the disc authoring process.
I would do this a little differently a DVD is always 720x576 in your case so you need to minimise the amount of resizing.
Go to the menu bar: Final Cut Pro > Easy Setup.
Choose DV-PAL Anamorphic. Click Setup. Go to the menu bar and choose Sequence > Settings. Click Load Sequence Preset. Click OK at the next window.
Back to the previous menu and change the Compressor and Field Dominace to match this...
When you drop your first clip into the Timeline, FCP will ask if you want to adjust the Sequence to match. Say NO.
You will notice that the 4:3 fills top to bottom, with pillars at the sides. You want space though at top and bottom. Double click the clip on your Timeline to send it to the Viewer. Click the Motion tab and adjust the Scale Parameter to a size you want - probaly about 80% or so. Don't worry about the rest - get editing. You can apply that size change to all the rest of your clips at once, later. Post again if you need help on that.
You only Export for Compressor to MPEG 2 if you are using DVD Studio. iDVD does all that for you automatically - just give it a 1:1 ProRes copy of the finished edit.
Read the DVD Studio Pro Manual. It can be found under the Help menu bar item, or here: http://documentation.apple.com I taught myself DVD Studio in about half a day using that resource.