capacity of rear diff? 12 bolt please hurry!

[hjalbert]

Im supposed to be driving this thing to la, then to az today need help!well, i couldnt get that doggone plug out, it had been broke several times in the past it looks like, and nothing i have will get a grip on it, so i took off the rear diff cover, and drained all the old nasty fluid, it was kindy muddy, hehe, any way, i need to know how much to put back in, because i am going to be filling it up through the vent hole, (that can be done right, as long i as know how much it is supposed to hold?) by the way, in case needed to know, this is a 69 gmc 4x4, i think it is full time four wheel drive, but i am not real; sure, any way, it has 6 lugs, and the back diff has twelve bolts. the plug in on the front of the diff, on the passenger side, and there is no drain hole, so the help would be appreciated. Harry