Only vent on a toploader I know of is on the top cover and that must be in working order or you can push oil out during operation. Since you are referring to this as a project I would assume you haven't driven it though. The front driveshaft yoke is hollow for a good portion of course but I don't know whether there's a passage where oil could seep all the way to the end to come out at the u joint. It seems unlikely, but if the trans is over full and/or there is enough play between the output shaft splines and the inputs on the yoke then oil could get past. I've got a spare DK yoke at home, will have to check that out of curiosity. Someone else probably knows for sure.
