Saw this post from days gone by. I work for Auto Gear in Syracuse and we produce and sell muncie parts. Heres the deal; the case will hold a quart and a half. My SuperCase will too. If you add a quart and a half, the trans will run hotter and eventually the oil will work its way out the front drain hole and if it doesnt drip on the ground as GM designed it; it will end up on the clutch...bad news
We; and most of the builders reccomend a quart.
Additionally; we ONLY reccomend NON synthetic NON blended 85w90 or the like WITH the GL-4 designation or GL-5 with the MT1 listing. These are safe for the old school synchros. The brand we recommend is Valvoline in the white bottle 85w90 or whatever it is. The blue bottle is a blend and has caused problems in the past. Synthetics are TOO slippery for the synchros to use as a friction brake and they end up clashing...
This isnt a advertisement for my company; Im trying to save you guys on parts, although if you have a gearbox question, just ask