Two things----------carb angle is to the ground (mother earth?) Driveline angle is to the crankshaft, tranny out put shaft, rear end pinion centerlines and is to accomadate the necessary movement of the needles in the U joints to insure long life versa a short notching session--
The timing of the joints and workable angle range is necessary for a smooth vibration free operation.
The general rule of thumb about the pinion being a degree or so more down than the trans output is to accomadate the torque rotation moment on acceleration to an equal angle with the front shaft joint.


The issue of carb base angles----------manufacturers use some of the same base engine packages in cars to heavy duty trucks-the taller (and sometimes shorter wheelbase) of 4 x4 s will require a steeper down angle toward the rear pinion than say a Camaro or mustang--------in order to get the carbed engines to run smooth and effienctly they needed to be tipped to maintain float bowl fuel levels thru out a varing range of acceleration, cruise and deceleration /and/or heavy braking---------


So, if you take a 460 from a Lincoln you will probably have a different manifold carb base angle than on a 429 sourced from a heavy duty truck------


If your carb base is at an extreme angle and you end up with back firing on acceleration, blubbering /stalling on decal, you might need to put an angled base plate under the carb or take off the intake and machine the carbsurface a few degrees---------or yu could remove the seats, cut the floor boards out and remount the engine /tranny till the carb is level and then put up with vibration on acceleration, rapid U joint wear, occasional tossed drive shaft, etc, etc


Build the drive line angles/timing and level the carb if necessary last