Nice job and very creative thinking but....
I hate to be the bearer of bad news... I really do... but the factory spare tire mount absolutely, really, cannot withstand the additional weight of that full jerry can even while just offroad. In fact, especially when offroad due to the additional stress that the bumps will create. That the jerry can is further away from the mount that the spare tire is gives it enormous leverage
against the spare tire mount. Eventually, the mount will crack & fail. Simultaneously it will put a lot of stress on the door hinges and the door won't close properly after a while.
I had the factory mount crack & fail just under the added weight of a 33" spare tire which is nothing, leverage-wise, compared to the added strain a full jerrry can creates.
Not to mention if anyone in a truck rear-ended you, I'd hate to see the results with them hitting a full jerry can first.