42RH / A-500 in a Duster with pictures.

Will be doing this very mod with a 42RH in a 69 barracuda. If I can suggest, you probably need to strap the cutout to box it in. I am going to use 1/8" steel flat stock bent into a horseshoe shape. Mine will probably be easier to weld in since I plan on modding the crossmember when I have the floor pan out for replacement.

As far as the down angle on the trans. Your carburetor mating flange on your intake should sit level. If you notice, intake manifolds have a slightly wedge shape from front to back because the driveline angles down and yet the carb sits level.

My recommendation is level the car front to rear on jacks with bubble levels taped to the doors rocker panels with the aluminum sill plates removed. With the car leveled, then remove your carburetor, and tape a bubble level to the carb flange on your intake manifold. Now set the angle of the drivetrain where the bubble on the carb flange is level. Now build your mount to that height.

If I am wrong here, please someone step up and provide other information since this was how I was going to go about it with the trans crossmember mount I will be making.

Hope this helps
Matt