I strongly advise against letting a conversion program make AC3 out of the DTS-HD MA. Dolby Digital encoding has several user-defined parameters which I specifically set a certain way to optimize the sound quality as well as to maintain the integrity of the mix. It is quite likely that the results will be subpar if this kind of transcoding is performed, because it won't follow my intentions and the result will not match what it is supposed to sound like. The level and relative phase of the surrounds and LFE can be harmed significantly if this is done incorrectly.

Also, the official Dolby encoder has a more capable algorithm than any freeware version, so my AC3 will definitely be better. As has been stated, remux the video and audio into the required format without re-encoding either in order to maintain quality and deliver the proper results.