With the mainshaft rebuilt, it's time to assemble the transmission. One BIG gotcha is that the heavy mainshaft (shown here dangling), the heavy countershaft, and the reverse idler gear (which must have been forged in Hades), must ALL be installed into the rear housing at the SAME time.
There is an "official" tool to aid in aligning all the gears so they can be inserted into the rear housing, which is basically a fixture upon which you set all the components vertically, and then drop down over that set the rear housing. I have no such fixture, so I used the front bell housing as a fixture of sorts
What's not shown in this picture is the next step, which involved my letting the engine hoist start a very slow descent while I quickly grab the countershaft, add the input shaft while hoping the 4th gear synchro doesn't fall off (it did many times), align the input shaft synchro keys, and align all components as they slowly descend into the housing.