Stub,
The layshaft, with the roller needles, thrust washers, and dummy shaft, is the first thing into the box, though I suppose you could install some of the reverse details as well. Lay it into the bottom of the gearbox (hence its name) and leave it there untill the mainshaft is installed. Then, raise the gearbox to a vertical position to more easily jockey the laygear/shaft with thrust washers into position. While doing this, be sure not to allow the dummy shaft to suddenly fall out of position, thus letting the rollers fall into the space where the shaft belongs. I usually "stack" the new shaft into the hole so that it would follow the dummy in this case. Otherwise, you'll get even more practice assembling this little beauty!!
One last hint. I've seen too many times where the rear flange has been tightened into place trapping the bulky felt "seal" between itself and the rear bearing and guard washer. Soon thereafter, the felt deteriorates, the end play becomes excessive, and jumping out of gear ensues. If that's all that happens, you have been living right!! Massive failure is also a possibility.
I hope we've answered your questions adequately. If not, do repost. This box is not a bad piece of equipment for its day, but like the engine in a T Type, it demands exactness and attention to detail if it is to give good service. There is no "That's good enough." here. When you have it right on the money, that's just about adequate.
Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon