You could author the MPEGs in something like TMPGEnc DVD Author (if they're already compliant!) and save, ignore filesize warning. Then open the authored DVD files in DVDRebuilder and re-encode at half D1. If you use Cinema Craft Encoder, I'd expect it to take 8-9 hours or so on a fairly recent machine, perhaps double that with QuEnc. That's assuming you don't fiddle with the defaults. Quality should be acceptable if the source files are good. That's the method I'd prefer, but there are many other ways to do it.
If the files are NOT compliant, you should re-encode to half D1, in which case you might as well get the bitrate correct without having to recompress later. Six hours would run about 1,560 kbit/s video if you used AC3 at 128 kbit/s. If the source files are crappy, better to just go with VCD resolution. Good luck.