As a practical matter, in original vehicles like early 50's Ford PU trucks and cars the bottom of the frame was level with the ground as built. So if that is what you are working with then set the frame on 4 equal height jack stands on a level floor and then level the carb flange. But these days with so many combinations of things like mono springs, IRS conversions dropped axles etc that may not be sufficient. Of course it is not completely necessary to get the carb level to run the vehicle but it is very helpful when setting the float levels.With regards to the drive shaft and rear axle alignment, the ideal design is that the centerlines of the engine, transmission, drive shaft, and rear axle pinion all be in line. Obviously when the vehicle goes over a bump or carries a different than expected load, the U joints come into play and allow the movement of the rear axle. If I remember correctly from machine design of 40 years ago, if the centerlines (in this case of the trany and pinion) are not aligned then they need to be parallel. Also the centerlines of yokes on the drive shaft must be in the same plain and parallel. This prevents a vibration.
I have not had very many drive shaft problems over the years and there are undoubtedly people reading this site that have practical experience in these matters. They are encouraged to jump in.