Problem:
convert a PAL 5.1 AC3 DVD to NTSC 5.1 AC3 DVD
I did my homework and could not find products (until now) that would accept AC3 5.1 or wouldn't down convert it to 2 channels AC3.

Solution:
DVD Decrypter to get your DVD to hard-disk.
DVD2AVI (Demux all tracks to AC3)
TMPGenc (set parameters for DVD to convert to PAL, then load unlock, and set to ES Video only).
DVD-Lab import the m2v file generated by TMPGenc and the AC3 file generated by DVD2AVI. Cut your DVD (may be IfoEdit to check it, if you don't trust me).

If your DVD is a DVD-9, some more work needs to be done, documented elsewhere on this site.

Conclusion:
I own Ulead, and made the big mistake to buy the AC3 PowerPack ($199). Useless, as it downconverts to two channels stereo. When I sent an e-email to technical support I got an arrogant response pointing me to the specs in Ulead Web page. I did my homework and tried some free products (e.g. ReMpeg,..). None can do it, except may be the open source FFmpeg, after some enhancements. Working in software development I don't want to spend time on it. DVD-Lab is $99, and that's the only product that I found that can do it. Don't buy the AC3 Powerpack unless you got a real need.
Last, DVD-Lab is slow in muxing (and I got a 2.8G processor), but it does the job, so its worth it.