Sorry, just not possible to fit on a DVD5 6 hours at full D1 and be even remotely "okay". That would mean a video bitrate around 1,500 kbits/s, barely more than VCD standard. (You could squeeze out only slightly more at the expense of the audio). Just try to spread that low a video bitrate out using a resolution 4 times larger- no way, no how. If you have a small TV and an uncritical eye, half D1 might be just watchable. Or else we're not talking about 6 hours, but considerably less.
Not to be pedantic, but MP2 is DVD standard (only in PAL-land, although most NTSC players will accept it) , not MP3. Otherwise, AC3 is it.
Hey, don't believe me, try it. Reauthor 6 hours worth in DVDShrink. Bleh. Or save with no compression and re-encode with DVDRebuilder. Still bad. Then try DVDRB at half D1. Marginal.