Scanlines in Direct 3D 'problem'

[hi_resurrection]

I wonder if someone can tell me how to put a game in direct 3D mode (1792x1344 resolution) and still use 75% scanlines simultaneously. Whenever I try I get a letterboxed screen or borders on all sides (with the game image centered).

 

Is it possible to do this? I always use max resolution/RGB sharp, but for a certain game where I want scanlines I don't want the blurry default image 'underneath' the scans.

 

Thanks.