I have the 901 with a back panel, and yes, it has ALL the capacity. All I would say is that while it is a chest rig, it is quite well padded and can still be quite bulky and sweaty (despite having "spacer mesh" type backing).
Most of the Spiritus Mk3 or 4 or the Haley DC3R type chest rigs will let you attach it to a MAP style assault pack (there's loads of knock offs floating around). The pack straps basically replace the chest rig straps.
Alternatively, take a look at the WTF stuff. They started out making tacticool gear for paintball so they have a number of air tanks specific pouches and packs (including a 13ci pouch that will sit horizintally on your belt if you want to get shot of the backpack completely). They do a range of modular chest rigs, and having used their gear, I would vouch for it over Chinese Spiritus clones!
https://www.wtfidea.com/air-tank-pouches/