Your car has a 10 degree approach angle if it's like mine, so about 8 degrees on the ramps would be safe. You can use Race Ramps made for trailer use to add to the effective length and lessen the angles.
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Using my small 18' gooseneck with a 2-foot dovetail, the ramps don't need to be overly long (can't recall what length I use for that car). It helps a lot if they are knife edged, by the way.
Maybe more importantly, if you go with drive-over fenders, is that they are not too tall for the doors to be opened. They need about nine inches if memory serves me