I could be wrong here, but don't most T-cases just fill up to the fill plug like manual trannys and rear differentials. I had a Wagoneer for a while and thats what that did... just filled it up to the fill hole and that's it. I think that was the NP203 but I wouldn't think they're much different in that respect. :dunno:
Are you asking about an NP205 as the subject line states, or the NP203 gear reduction box as indicated in your message?Its kinda hard to know what you are really asking for. If it is the NP203, the fill plug on the original is located on the gear drive part that is discarded when you make the doubler. Consequently the NP203 gear reduction box does not have a fill plug, and one must be added. The easiest way do do this is to weld in a bung into the NP203 side inspection plate. IIRC, the ORD doubler kit includes a new side inspection plate with a fill plug. If doing it yourself position the fill plug level with the idler shaft. I can't remember how much lube that actually takes, but I think it is a bout a quart and a half.