That REI bag is probably a 30 degree....you are going to need a new bag. Sorry.
I know you like how the x-frame pack carries a heavy load. But, if you look at the physics of packs, you will see that it all comes down to the pack's ability to transfer the load/weight to the hip belt. The x-frame packs are the most efficient at doing this, for obvious reasons. But, that does NOT mean that UL packs are far worse. Just slightly not as efficient. Everyone raves about Zpacks ArcHaul and it is on my short list for big dollars for new gear. However, it obviously is not for everyone, or there would not be so many for sale used!! Do you know how to tell a good car model? By the lack of availability of used ones in the market(or their high resale price, i.e. Toyota)...so...take everyone's glowing rec's for anything with a grain of salt. If there is an unusual or disproportionately high rate of a certain item for sale in the used gear market, especially at low prices, its a good sign that the design isn't working out for many folks and that the cognitive dissonance is so high that they will part with a barely used item for far less than the purchase price. I think many people thought the ArcHaul was a cure-all for their issues with packs, and it really isn't. Its just a lightweight Cuben Fibre sack suspended on some carbon fibre rods that connect to the hip belt. Lightweight being the key for me. Also, it has a very unique design whereby you can actually alter the curve of the frame of the pack to keep it off of your back. Infinitely adjustable. That, and the weight, are the big selling points for me. Since you want the pack off your back, I'd say look for a used ArcBlast or ArcHaul on the used gear market. If that is too much money, there are old model packs that are new for sale. Right now you can get the 2014 50L Fusion by SixMoonDesigns for a Benjamin. Brand new from the manufacturer. No warranty, though...
https://www.sixmoondesigns.com/collections/bargain-bin
It takes some internet hunting and forum perusing to find the dealios...just sayin...
You won't be carrying two gallons for very long. Two gallons weigh what? 16.6 pounds....unless you've got an ultralight pack with a load limit of 25 pounds, any internal frame pack will accept the load...Even for the very limited occasions when you have to carry a heavy load, you don't want an overbuilt overweight pack for all the other times....look at the big three thousand mile picture....