Put a new seal into the front cover, bolt it to the transmission, and safety-wire it in place.
The seal on the tailhousing should be replaced at every rebuild. It can be tapped into place with a dead-blow hammer.
Before the tailhousing can be bolted back on, the reverse gear must slide in place. Don't forget to make sure that the collar on the gear slides inside the slot on the reverse selector lever.
Now you can finally bolt the tailhousing back to the main case. Carpenter uses blue Loctite on all these bolts.
Before repositioning the sliders, give everything a thick coat of oil to protect the gears the first time the transmission is used. The case will be filled with lubricant later, but that won't distribute lubricant to all the necessary places; the last thing you want to do is to put your new transmission into gear with the gears dry.
Carpenter pressure tests every transmission after it is finished. This is cheap insurance to make sure it won't leak after the team installs it in their race car.
Reinstall the side cover, making sure that the gear levers are properly engaged to the shifter forks.