I believe that the procedure is the following:

1. Capture at 720x576.
2. Crop 8 pixels from each side to account for overscan, leaving you with 704x576.
3. Use PAL's 12:11 PAR to get square pixels, so (704 * 12)/11 = 768, leaving you with 768x576.

It's preferable to crop and resize in one invocation so as to not cause sampling issues. The following would do the trick:

capture.Spline36Resize(768, 576, 8, 0, 704, 576)

Edit: Make sure to not do any more horizontal cropping after this!

Edit 2: Also, if you're interested in cropping out the black borders on the top and bottom, please let me know so that I can modify the above code.

Edit 3: Do you want me to give an example using PAL GOUT? I can include information on how to make it anamorphic widescreen as well.