Well, I use the Vans standard (almost 100" forward of the wing? I forget.) as the moment arm zero point, and my weight and balance data shows the very back of the baggage area (F1006) as being at 189". But surely you have similar data for your airplane? I'd have to remove the baggage area bulkhead to measure the distance between F1006 and F1007, or where ever you plan to put the tank. You can probably get close by measuring to the rivet line, externally.
I'd urge caution. It's pretty easy to get -10's too far aft when fully loaded, especially if low on gas; putting an O2 tank that far aft won't help any. I'd also give some thought to crash-worthiness. Your mounting system should hold the weight of the tank times about 10, in the forward and down directions.