Joe,
Buy 3 quarts.....it'll take just a hair under 3, depending on your vehicle's stance, to get fluid to drain back out the filler hole. That way, you know that the rear tailshaft bushing and slip yoke are getting plenty of lubricant.
If you still plan on using ATF in your Non World Class transmission I'd suggest Royal Purple's Synchromax which is formulated for manual transmissions where the mfgr suggests ATF.
Personally, I've run gear oil in my 54 Chevy truck with a used NWC 1352-145 T5 for the past 14,000 miles with no issues. I've also been running the RP Synchromax in my Hot Rod with a T5 "Z" Hybrid NWC behind a built 355 SBC with no issues. Makes you think that the NWC brass synchronizers/bearings/bushings/gears can handle both weights and that the heavier lube might be better in warmer climates. Hope this helps you get your truck back "on the road again."
Dave
