Its one of those things where you have to almost relax...lay still and experiemnent with how much pressure you apply while testing. The design of the tailcone bushing is a "mushroom" shape so it has a fairly soft portion that actually makes contact all around inside the socket in the trailing crossmember. So what you are looking for is movement of the tailcone 360*...equally with just enough movement in each direction to represent the compression of the rubber mushroom on the end of the bushing.
In reality....if your upper hanger bar bushings are in good shape.....and you have not removed them for any reason and lost or altered teh pack of shims under each bushing.....and you are carefull to reinstall the rear hanger bar just as you found it....your alignment will generally be fine.
99% of all 411/412 owners will never know that there are shims under the upper hanger bar bushings until they go to change one. So if yours has original bushings and they are not torn....this area is usually ok. If only one side of the upper hanger bar bushings gets changed...the alignment will usually be off. Always change teh bushings in pairs.
The other problem is at the rear hanger bar.
If you must remove the engine and your alignment is good already, the proper method is to remove the two horizontal bolts at each end that are stacked one on top of each other and locked with peened over metal plates.......being sure to mark where they are in their side to side slotted holes first. The movement at the tailcone that these side to side slotted bolts create at the tail cone is to slew it either to the 3:00 position (drivers side) or the 9:00 position (passenger side) in the socket in the trailing crossmember.
Then....remove the lower/outer nut... ONLY....of the vertical bolt that goes through each end of the hanger. Make sure the nut above each end of the bar is tight and locked in position with its jamb nut.
This jamb nut adjustment sets the height in the vertical plane of the hanger bar. The movement it creates at the tail cone in the crossmember socket is to drive the tailcone to either the 6:00 position (down) or the 12:00 position (up).
If you run out of side to side adjustment at the slotted bolts.....center them in their bores.....then look at the actual mounts in the body on each side. These are the plates that have the fork that the hanger bar slips into...and also contain that vertical bolt with the jamb nut adjustment. They are bolted to the body with two 13mm bolts....that screw into captive nuts in cages in the body. Make sure tehse captive nuts are clear of crud thrown up from the road so the position of the outer body/hanger bar mount can be moved. These captive nuts allow you about 1/4" movement 360*. So if you run out of side to side adjustment at the hanger bar bolts....loosen the outer mounts and slews teh drivetrain package where you need it to be...and start adjusting again.
the last major propbelm is when you replace the center rear hanger bar mounts that contact the engine. Some aftermarket mounts are too hard...some are ill fitting, some are too soft. In teh Haynes manual for the late bus with type 4 motor...there is actually a small drawing that shows how much weight crush deflection should be on these center hanger bar bushings. It is only marginally useful as the hardness of bushings made now are rarely equal to factory.
As long as the tailcone bushing is dead centered in its bore and the pressure to move the tailcone is equal any direction you push...and you can push and pull at the center hanger bar bushings in the rear and get some slight up and down flex....youshould be good. Ray