360 Magnum with 42rh install angle question.

IMO, in an A-body
you will need to get the front of the engine as low as is possible, and the back of the trans up as high as is possible, going towards but not past, parallel to the rockers;
then deal with the rear driveline angles; which are; driveshaft to trans and driveshaft to pinion, and have nothing to do with the horizontal.
IMO having the engine at ~2* front high with the rockers leveled in side view, is gonna be close enough to work with. I would mock it up from there and see where the back comes in.