Faceplated 833 4 Speed

Your assembly technique is the same as I have always done for nearly 50 years.
My bench has a hole in it and I stand the case on it's face as soon as the cluster is laying in there, with the input dropped into the hole. Then I drop the assembled rear into place steering the nose into the input rollers . Then I rotate the tail housing about 180* from normal to expose the cluster pin hole. Next I pull the cluster into position and drop the pin in. Finally, I rotate the tail back to normal, and install the bolts. Yeah you have to be careful to not tear the gasket, which I lightly grease to the the case.
As for the washers and rollers, I use wheel-bearing grease to hold everything in position, no arbor.
I've built hundreds like that, possibly thousands.