You are correct that you should load the crown wheel away from the pinion when checking contact patch. But you don't want to introduce a preload on the big bearing when it is in service.
I'm also used to finding the big bearing being an intimate fit on the ring gear and in the housing cover. None of the assembly is floating.
Remove the pinion gear, drop the crown gear mounted to the shaft into the needle bearing. It comes to rest on the brass thrust bearing.
If the crown gear mounted to the shaft is installed with the roller bearing seated on the shaft shoulder face and fully seated in the cover, the opposite end will not be touching the brass thrust bearing (on my gear box) .o52 inch. With the two paper gaskets and .050 shim.
If the roller bearing is installed in the cover first, crown gear and shaft dropped into the needle bearing and resting on the thrust bearing, Cover with bearing installed on crown gear shaft, there is .052 clearance between the inner roller bearing race and the shaft shoulder face on the crown wheel shaft when the cover is installed. With the two paper gaskets and .050 shim
The crown gear shaft has to be sandwiched between the brass washer and the inner race of the roller bearing seated on the crown wheel shaft shoulder face with some amount of clearance and no side loading. .052 seems a little excessive with the shim installed.
The procedure of installing the pinion and then raising or lowering the crown wheel with shims (and adjusting the pinion) raises or lowers the crown wheel shaft off or onto the thrust washer. Allowing the shaft to float if the clearance is excessive. To me, it looks like it is a combination of adding or removing brass thrust washers on the needle bearing end and shims in the cover to get proper positioning on the crown wheel and pinion and to make up for the end play of the crown gear shaft.
If there is clearance between the end of the crown wheel shaft and the brass bushing, the crown gear will be using the pinion gear as a stop instead of the brass bushing at the end.
Installing the big bearing in the cover fully seated, the big bearing on the crown wheel shaft fully seated. When the cover with the crown wheel is installed using 2 paper gaskets to get .0025 end clearance between the thrust washer and the crown wheel shaft end. Recheck the pinion to crown wheel fit. A paper gasket is .0075 thick. If I only use two, I get .0025 clearance between the end of the shaft and bushing. Since the crown wheel is pushed toward the big roller bearing under load, this small amount clearance should no allow excessive pressure on the pinion and crown gear if the shaft moves toward the thrust washer when not under a load.
The brass thrust bearing is there to stop the crown gear and the pinion gears from being too tight or loose. The two gears are not suppose to be the side thrust stop when meshed. The inner race on the big gear and the thrust ring are.