I only use Splinexx for downsizing. (I find Spline16 faster than Spline36, but perhaps that is just my perception.) I use Lanczos3 for upscaling. Is that not what we are doing here? I thought the recommendation was that it was that way round, but perhaps I am wrong.
Personally, I would crop eight pixels from the left and eight pixels from the right and author a 704x480 DVD, but I suppose it is just preference. I have done this numerous times recently and the result has always played perfectly. I have even come across commercial DVDs that are 704x480.