"My question is what would be the best way to mount a SBC in a Model A frame"

Oil pan down, carburetor up.

Sorry, I couldn't help it

As far as motor mounts, you may have to fabricate some to custom fit the car. It's no big deal. Just get some cardboard and cut it with scissors or razor blade to the proper design, then transfer your pieces to 1/8" cold rolled steel and weld it up.

To get the proper tilt on the motor, have the car at ride height and level the motor by the carburetor pad on the intake manifold. This will angle the motor at 2 1/2 to 3 degrees, depending on the angle cut on the manifold. Use it to also level the motor side to side. It's ok if you have to move the motor to the passenger side a little to miss steering or headers, just make sure the centerline of the motor/trans is parallel with the centerline of the car. In other words, if you move the motor from centerline, DO NOT point the trans tailshaft at the center of the differential, point it straight back, no matter where it points to. The u-joints in the driveshaft will take care of the misalignment. That's their job. The pinion in the differential should point up about the same amount as the motor points down, 2 1/2 to 3 degrees for a street cruiser. If you will be drag racing some with the car, you might lay the pinion in at a lesser angle, maybe 1 to 1 1/2 degrees up.