I recently had my T56 out of a 2002 F-Body rebuilt to Stage 2 specs by Jason at TDP.

Installed the flywheel, clutch, pressure plate, all clutch hydraulics, and then mated up the trans to the motor. Went to fill up the transmission with Dex III ATF through the shifter hole, and it just started pouring out the back of the output shaft. There looks to be a brand new seal right there, but I do not have an output slip yoke installed yet. Is the slip yoke responsible for keeping the fluid inside the trans? I already have a slip yoke on order since I’ll obviously need it anways.

 

If you have it at a slight angle, yes it will pour out, hence the seal and slip yoke. More so if you fill from the shifter hole, it is almost a straight shot out the back of the transmission. Shifter hole is he easiest place to add the correct amount of fluid but you need the slip yoke in place.