Eric,
The factory installed the shims between the crankcase and the bearing. There are slingers (in some designs) between the bearing and the crankshaft, but these are not intended to be shims. Presumably Kingswood (and other factories) did this to protect the thin and delicate shims. If they were installed on the crankshaft they would be more prone to damage during assembly. The crankshaft journal has a fillet between it and the throw to avoid a stress riser. If the shim registered on the crankshaft, the hole through would have to be bigger than the journal so that it would clear that fillet. During assembly you would have to be careful the shim rode up that fillet and registered on the flat face, else it might get pinched and not allow the bearing to seat fully. Also they might have figure the crankshaft shuffling back and forth the amount of the running clearance would unduly hammer or at minimum compress pinch the thin shims as the crankshaft flexes, so behind the bearing they are static as well as better protected. It seems to be normal practice, not just with Douglas.
I actually do not remember a slinger(s) on my 1936 Aero, but it has been a while since the engine was assembled and I did not take a photo of the assembled crankshaft that I can refer to. Most designs have a shallow spigot for the slinger to register on, just for the purpose to keep it off the journal radius. I do have a picture of the disassembled crank, and I do not see a spigiot for the rabbet so the must have done without.
Anyway, since the shim is behind the bearing, it has to be installed before the bearing is shrunk into the case. And since the traditional method involves some trial and error, the shim (and bearing) needs to be removed/installed several times until one gets the shim amount correct. I think that also answers why you cannot us the aluminum bar to just gauge the distance between the bearing races once in the crankcases.
One could divide the shims between the timing and flywheel side, but to be honest I cannot recall ever finding shims anywhere but on the flywheel side. The amount is so small, that it is neither enough to throw the rods significantly off center, or enough to correct any misalignment of the conrods. The rods rarely are exactly centered nor is it critical (allowing for the usual no rubbing, turns over freely, etc.) The flywheel side crankcase is easier to heat up to fix the bearing; easy access inside and out for the application of heat. I suppose that is why the shims are installed on that side.
-Doug
[fix typo. 14Nov19. Doug]